UCSB  LIBRAKf 


MODERN   THEORY 

OF 

PHYSICAL  PHENOMENA 


MODERN  THEORY 

OF 

PHYSICAL   PHENOMENA 

RADIO-ACTIVITY,  IONS,  ELECTRONS 

BY 
AUGUSTO    RIGHI 

PROFESSOR  OF  PHYSICS  IN  THE  UNIVERSITY  OF  BOLOGNA 

AUTHORIZED  TRANSLATION 

BY 
AUGUSTUS   TROWBRIDGE 

PROFESSOR  OF  MATHEMATICAL  PHYSICS  IN  THE 
UNIVERSITY  OF  WISCONSIN 


KTefo  ff  0tfe 
THE    MACMILLAN   COMPANY 

LONDON  :   MACMILLAN  &  CO.,  LTD. 
1909 

All  rigfiti  reterveJ 


COPYRIGHT,  1904, 
BY  THE  MACMILLAN  COMPANY. 


Set  up  and  electrotyped.    Published  December,  1904.    Reprinted 
January,  1909. 


Notfoaoto 

J.  B.  Cashing  Co.  —  Berwick  <k  Smith  Co. 
Norwood,  Mass.,  U.S.A. 


PREFACE 

IT  is  with  a  certain  trepidation  that  I 
have  entertained  the  proposal  of  an  English 
translation  of  an  entirely  unpretentious  book, 
which  is,  in  fact,  only  an  extension  of  a 
chapter  added  to  "  Telegrafia  senza  Filo " 
(Telegraphy  without  Wires). 

The  fact  that  this  book  has  gone  through 
two  Italian  editions  in  a  very  short  time 
certainly  shows  that  my  work  has  not  been 
useless  in  my  own  country.  But  to  issue  an 
edition  in  the  language  of  those  illustrious 
men  to  whom,  above  all,  we  owe  the  ad- 
mirable theory  which  forms  the  subject  of 
this  little  book  is  quite  a  different  thing. 

That  which  has  weighed  with  me  in  my 
decision  was  the  thought  that  this  English 
translation  might  at  least  serve  to  show  with 
what  great  favour  the  development  of  the 
ideas  concerning  the  first  cause  of  physical 


vi  PREFACE 

phenomena  has  been  received  in  Italy,  and 
also  to  show  how  fully  the  works  of  those 
philosophers  to  whom  this  development  is 
due  are  appreciated  in  this  country. 

AUGUSTO  RIGHI. 

BOLOGNA,  ITALY, 
August,  1904. 


TRANSLATOR'S   PREFACE 

FOR  more  than  twenty-five  years  Profes- 
sor Righi  has  been  an  indefatigable  investi- 
gator and  constant  contributor  to  electrical 
science,  and  has  attained  a  foremost  rank 
among  Italian  scientists.  While  he  is  not 
one  of  the  relatively  small  group  of  investi- 
gators to  whom  we  owe  the  extremely  im- 
portant theory  outlined  in  this  book,  he  is, 
nevertheless,  admirably  qualified  to  discuss 
and  explain  the  theory,  both  by  reason  of 
his  deep  insight  into  electrical  phenomena 
and  because  of  his  ability  to  explain  intri- 
cate physical  processes  without  the  aid  of 
mathematical  formulae. 

As  Professor  Righi  states  in  his  preface 
to  the  first  edition,  the  Italian  original  was 
written  more  with  the  object  of  interesting 
the  greatest  possible  number  of  readers  in 
this  new  and  important  branch  of  physics 


viii  TRANSLATOR'S  PREFACE 

than  as  a  book  of  reference  for  physicists. 
With  this  end  in  view  the  subject-matter 
was  presented  in  an  elementary  form,  and 
the  book  met  with  a  very  marked  success. 

I  have  made  this  translation  believing  that 
there  are  at  least  as  many  English  as  Italian 
readers  to  whom  an  elementary  treatment  of 
the  electron  theory  as  it  stands  at  present 
will  be  acceptable. 

Professor  Righi  has  read  the  proofs  of  the 
translation,  thus  insuring  its  accuracy,  and 
has  kindly  provided  me  with  a  special  preface 
in  English. 

AUGUSTUS  TROWBRIDGE. 

MADISON,  WISCONSIN, 
September,  1904. 


CONTENTS 


PACK 

INTRODUCTION  .  .        .        .        .       xi 


I.    ELECTROLYTIC  IONS  AND  ELECTRONS      .        .  i 
II.    THE    ELECTRONS   AND    THE    PHENOMENA    OF 

LIGHT n 

III.  NATURE  OF  THE  CATHODE  RAYS     ...  28 

IV.  THE  IONS  IN  GASES  AND  IN  SOLIDS         .        .  40 
V.    RADIO-ACTIVITY 54 

VI.    MASS,  VELOCITY,  AND  ELECTRIC  CHARGE  OF 

THE   IONS   AND   OF   THE   ELECTRONS        .            .  Io8 

VII.    THE  ELECTRONS  AND  THE  CONSTITUTION  OF 

MATTER 141 

BIBLIOGRAPHY     .       » 153 


INTRODUCTION 

A  NEW  and  interesting  branch  of  science 
has  been  formed,  partly  as  a  result  of  the 
numerous  recent  experimental  researches  on 
the  electric  discharge,  and  partly  as  a  conse- 
quence of  the  discovery  of  radio-activity  and 
new  phenomena  in  magneto  optics.  At  the 
same  time  these  researches  have  caused  a 
theory  to  spring  up  which  harmonizes  all 
the  facts,  and  which  has  profoundly  modified 
the  dominant  ideas  concerning  the  imme- 
diate cause  of  electrical  phenomena,  and  of 
physical  phenomena  in  general. 

When  the  old  hypothesis  of  the  electric 
fluid  was  abandoned,  chiefly  because  of  the 
disinclination  to  admit  "action  at  a  dis- 
tance/' it  seemed  for  a  time  as  though  Fara- 
day's ideas,  formulated  later  by  Maxwell, 
must  lead  to  a  new  concept  regarding  the 
cause  of  electrical  phenomena,  since,  accord- 
ing to  Faraday,  the  ether,  and  not  the  so- 


Xll  INTRODUCTION 

called  electrified  bodies,  is  the  seat  of  the 
phenomena.  But  the  impossibility  of  find- 
ing a  satisfactory  mechanical  representation 
of  the  supposed  elastic  deformations  of  the 
ether,  to  which  were  attributed  in  Maxwell's 
theory  the  apparent  forces  at  a  distance,  and 
the  necessity  of  admitting  at  any  rate  the 
existence  of  an  entity  distinct  from  the  ether 
and  from  matter,  made  it  soon  apparent  that, 
even  under  the  new  order  of  ideas,  the 
conception  of  the  nature  of  electricity  still 
remained  obscure. 

At  present  a  new  evolution  is  being  ac- 
complished, since,  without  knowing  anything 
more  about  the  first  cause,  an  atomic  struc- 
ture is  attributed  to  electricity.  This  new 
conception,  suggested  by  the  studies  men- 
tioned farther  on,  already  shows  promise  of 
becoming  as  fruitful  as  the  analogous  one 
which  has  long  been  admitted  regarding  the 
constitution  of  matter,  inasmuch  as  it  per- 
mits us  to  place  in  reciprocal  relation,  often 
even  quantitative,  phenomena  which  seem 
utterly  different  and  independent  of  each 
other. 


INTRODUCTION  xm 

What  the  electrons  or  electric  atoms  really 
are  remains  a  mystery;  but  in  spite  of  this 
the  new  theory  may  perhaps  acquire  not  a 
little  importance  in  the  future  even  from 
the  philosophic  point  of  view,  since  it  points 
out  a  new  mode  of  considering  the  structure 
of  ponderable  matter  and  tends  to  bring  back 
to  a  single  origin  all  the  phenomena  of  the 
physical  world. 

It  is  indeed  true  that,  with  the  modern 
positivist  and  utilitarian  tendencies,  many 
people  do  not  appreciate  this  advantage,  and 
prefer  to  consider  a  theory  principally  as  a 
convenient  means  to  arrange  and  coordinate 
facts,  or  as  a  guide  for  the  investigation  of 
new  phenomena.  But  if  hitherto  men  have 
confided  too  much  in  the  power  of  human 
ingenuity  and  have  too  easily  believed  them- 
selves to  be  on  the  point  of  discovering  the 
ultimate  cause  of  things,  we  perhaps  fall  into 
the  contrary  excess. 

In  this  book  I  shall  set  forth  the  principal 
facts  which  have  led  to  the  electron  theory, 
and  I  shall  endeavour  to  make  this  theory 
clear  at  least  along  its  general  lines. 


THE  MODERN  THEORY  OF 
PHYSICAL  PHENOMENA 

CHAPTER   I 

ELECTROLYTIC  IONS  AND  ELECTRONS1 

THE  hypothesis  of  electrolytic  dissociation 
is  generally  admitted  in  order  to  explain 
electrolysis  in  accord  with  the  well-known 
laws  of  Faraday  which  this  phenomenon 
obeys.  Each  molecule  of  an  electrolyte  may 
break  up  into  two  ions;  that  is,  into  two 
atoms  or  atomic  groups  having  equal  charges 
of  opposite  sign.  Thus  when  a  solid,  such 
as  chloride  of  sodium,  or  common  salt,  is 
dissolved  in  water,  some  of  its  molecules 
undergo  dissociation;  that  is,  these  mole- 
cules cease  to  exist  as  such,  and  their  ions 
become  separated  and  free.  Owing  to  the 

1  The  numbers  in  parentheses  inserted  in  the  text  refer  to 
the  bibliography  at  the  end  of  the  book. 

B  I 


2   ELECTROLYTIC  IONS  AND  ELECTRONS 

invisible  molecular  and  atomic  motions, 
whose  energy  constitutes  the  heat  contained 
in  a  body,  these  ions  wander  through  the 
liquid,  without,  however,  any  given  direction 
of  motion  predominating.  In  the  resulting 
mutual  collisions  it  happens  at  one  instant 
that  a  molecule  breaks  up  into  ions,  at 
another  instant  that  individual  ions  recom- 
bine  into  molecules.  There  are,  so  to  speak, 
incessant  unions  and  separations,  in  spite  of 
which  the  number  of  dissociated  molecules 
remains  sensibly  independent  of  the  time. 

When  two  electrodes  connected  with  the 
poles  of  a  battery  are  immersed  in  the  solu- 
tion, the  ions  of  the  two  kinds  —  for  ex- 
ample, the  positive  ions  of  sodium  and  the 
negative  ions  of  chlorine  —  no  longer  wander 
at  haphazard  in  any  direction ;  but,  obeying 
the  electric  force,  the  first  approach  the  nega- 
tive electrode,  or  cathode,  the  second  the 
positive  electrode,  or  anode.  On  arriving 
at  the  electrodes,  the  ions  give  up  their 
charges  and  become  neutral  atoms,  which 
remain  free,  at  least  if  no  special  chemical 
action  takes  place  between  these  atoms  and 


ELECTROLYTIC  IONS  AND  ELECTRONS    3 

the  surrounding  bodies  (this,  by  the  way,  is 
precisely  what  would  happen  in  the  case  of 
the  sodium).  The  electric  current  in  the 
liquid  consists  in  this  transport  of  electricity 
brought  about  by  the  ions. 

Electrolysis  obeys  the  two  laws  which 
were  enunciated  by  Faraday.  The  first  of 
these  laws,  which  asserts  the  proportionality 
existing  between  the  quantity  of  electricity 
traversing  the  liquid  and  the  quantity  of 
matter  deposited  on  the  electrodes,  is  em- 
bodied in  the  statement  that  all  the  ions 
existing  in  the  liquid  possess  charges  equal 
in  absolute  value.  Thus,  in  the  case  of 
chloride  of  sodium,  the  ions  of  the  metal 
have  positive  charges  all  equal  among  them- 
selves, and  equal,  but  of  opposite  sign,  to  the 
charge  of  any  one  of  the  ions  of  chlorine. 

In  order  to  satisfy  Faraday's  second  law, 
according  to  which  a  proportionality  exists 
between  the  relative  chemical  equivalents  of 
different  electrolytes  and  the  amount  of  each 
decomposed  when  the  same  quantity  of  elec- 
tricity is  transmitted  through  them  all,  as 
would  be  the  case  when  all  were  placed  in 


4    ELECTROLYTIC  IONS  AND  ELECTRONS 

series  in  a  single  circuit,  it  is  necessary  to 
admit  that  all  univalent  atoms  possess  a 
charge  equal  in  absolute  value  to  that  of  the 
sodium  or  of  the  chlorine  ion :  that  all  atoms 
which  possess  twice  this  charge  behave  as 
bivalent,  etc.  The  following  example  will 
make  this  point  clear.  If  a  current  is  passed 
through  a  solution  of  the  chloride  of  copper 
the  molecules  of  which  contain  two  atoms  of 
copper  (univalent)  and  two  of  chlorine,  and 
also  through  a  solution  of  the  other  chloride, 
the  molecules  of  which  contain  one  atom  of 
copper  (bivalent)  and  two  of  chlorine,  there 
will  collect  on  the  cathode  of  the  first  solu- 
tion a  quantity  of  copper  double  that  on  the 
second  cathode,  although,  naturally,  the  quan- 
tity of  positive  electricity  transported  through 
the  two  liquids  will  have  been  the  same. 

In  1 88 1  the  illustrious  Helmholtz  pointed 
out  that  the  laws  of  electrolysis  suggest  the 
idea  that  the  electric  charge  pertaining  to 
any  valency  of  an  ion  may  be  a  fixed  quan- 
tity having  a  separate  existence ;  and,  since 
a  material  atom  is  a  fixed  and  determinate 
portion  of  a  certain  kind  of  matter  and  is 


ELECTROLYTIC  IONS  AND  ELECTRONS    5 

considered  indivisible,  it  is  thus  natural  to 
consider  this  electric  charge  as  fixed  and  in- 
divisible, all  the  more  because  a  quantity  of 
electricity  smaller  than  this  is  never  encoun- 
tered. The  charge  of  the  ion  (univalent) 
may  therefore  be  called  an  atom  of  electric- 
ity, or  better,  as  Mr.  Stoney  has  proposed, 
an  electron  (electric  ion). 

In  fact,  as  early  as  1871,  considerably  in 
advance  of  Helmholtz,  Weber  conceived  the 
idea  of  the  atomic  structure  of  electricity. 
This  famous  physicist  and  mathematician 
proposed  the  well-known  theory,  according 
to  which  electric  phenomena  are  due  to  par- 
ticles or  atoms  of  positive  and  negative  elec- 
tricity acting  on  each  other  at  a  distance, 
with  forces  depending,  not  only  upon  the 
distance  itself,  but  also  on  the  velocity  of  the 
particles  and  on  their  accelerations ;  that  is, 
on  the  manner  in  which  these  velocities  vary. 
Naturally  this  theory  in  which  action  at  a 
distance  was  still  admitted  has  nothing  in 
common  with  that  now  in  favour  except  the 
fundamental  concept  of  the  electric  atom ; 
and  although  Weber,  searching  in  his  theory 


6    ELECTROLYTIC  IONS  AND  ELECTRONS 

for  the  cause  of  the  forces  which  govern  the 
atomic  structure  of  bodies,  advanced  the 
hypothesis  "  that  to  every  ponderable  atom 
there  is  united  an  electric  atom"  (i),  the  re- 
lation now  admitted  between  ions  and  elec- 
trons seems  to  have  been  better  perceived 
by  Helmholtz. 

We  do  not  believe  that  the  atomic  hy- 
pothesis concerning  the  nature  of  electricity 
imposes  on  us  the  necessity  of  considering  it 
as  matter,  since  we  are  still  free  to  suppose 
than  an  electron  may  be  simply  a  special 
localized  condition  of  the  universal  ether. 
We  may,  on  the  contrary,  from  now  on  add 
that  instead  of  considering  electricity  as 
matter,  we  are  led  to  the  exactly  opposite 
hypothesis  that  the  atoms  of  various  bodies 
are  systems  of  electrons. 

When  the  ions  arrive  on  the  electrodes 
and  become  neutral  atoms,  the  electrons  en- 
ter into  the  circuit  to  constitute  the  electric 
current.  Now  it  seems  natural  to  suppose 
that  these  electrons,  instead  of  merging,  so 
to  speak,  into  a  homogeneous  whole  (the 
old  electric  fluid),  preserve  their  individual- 


ELECTROLYTIC  IONS  AND  ELECTRONS    7 

ity;  this  is  all  the  more  natural  because,  if 
they  are  to  pass  from  one  atom  to  another, 
it  is  most  probable  that  they  must  exist  mo- 
mentarily isolated ;  thus  the  electric  current 
in  conductors  would  be  nothing  else  than  a 
motion  of  free  electrons  across  interatomic 
space.  It  remains  undetermined  whether 
the  current  consists  in  the  motion  of  posi- 
tive electrons  in  one  direction  and  negative 
in  the  opposite  direction,  or  in  the  motion  in 
a  given  direction  of  one  of  the  two  kinds  of 
electrons,  say  the  negative ;  but  preference 
is  given  to  the  latter  opinion,  because,  while 
there  is  reason  to  hold  that  the  negative 
electrons  may  exist  in  a  free  state,  this  is  not 
true  of  the  positive  electrons.  Only  the 
former,  as  it  appears,  suffer  displacement, 
separate  themselves  from  ponderable  matter 
or  unite  with  one  another,  and  vibrate  in 
light  sources,  as  we  shall  soon  see.  There- 
fore, while  a  negative  ion  in  being  deposited 
on  the  anode  gives  up  its  electron,  a  positive 
ion  arriving  at  the  cathode  does  not  give  up 
the  positive  electron,  but  takes  away  a  nega- 
tive electron  from  the  cathode  itself. 


8         ELECTROLYTIC  IONS  AND   ELECTRONS 

Here,  then,  is  the  old  theory  of  the  electric 
fluid  in  a  certain  sense  called  back  to  life, 
but  profoundly  modified.  It  is  no  longer 
a  question  of  a  continuous  fluid,  but  of 
special  atoms  (the  electrons),  which,  however, 
as  has  already  been  observed,  are  not  neces- 
sarily to  be  considered  as  material  in  the 
ordinary  sense  of  the  word. 

Besides,  and  this  is  of  more  importance, 
we  do  not  attribute  to  the  atoms  of  electricity 
that  mysterious  faculty  of  acting  at  a  dis- 
tance, with  which  the  old  fluid  was  supposed 
to  be  endowed,  but  instead  we  suppose  that 
the  reciprocal  forces  between  the  electrons 
have  their  origin  in  the  special  elastic 
deformations  of  the  ether,  identical  with 
those  called  for  in  Maxwell's  theory  to  take 
account  of  the  electric  forces  between 
conductors. 

To  explain  the  phenomena  of  electrolysis 
it  is  sufficient  to  admit,  as  is  always  done, 
the  hypothesis  of  electrolytic  dissociation ; 
but  this  hypothesis  is  not  well  adapted  to 
explain  the  propagation  of  electricity  in 
gases  and  certain  other  phenomena.  How- 


ELECTROLYTIC  IONS  AND  ELECTRONS    9 

ever,  with  the  admission  of  electric  dissocia- 
tion, —  that  is,  the  separation  of  the  negative 
electrons  from  the  neutral  atoms, — we  assign 
a  reason  both  for  electrolysis  and  for  these 
other  phenomena  as  well. 

In  order  that  a  negative  electron  may 
separate  itself  from  a  neutral  atom,  energy 
must  be  expended  to  overcome  the  attrac- 
tion by  which  the  electron  is  held  to  the 
positive  ion,  which  is  what  remains  of  the 
atom  when  the  negative  electron  is  taken 
from  it,  precisely  as  it  is  necessary  to  fur- 
nish heat  energy  to  separate  the  molecules 
of  a  liquid  from  one  another  in  evaporation, 
or  as  it  is  necessary  to  do  mechanical  work 
in  lifting  a  weight  from  the  earth. 

The  energy  necessary  to  ionize  or  dissoci- 
ate an  atom  naturally  varies  according  to  its 
chemical  nature.  Experiment  indicates  that 
this  energy  is  a  minimum  for  the  so-called 
electropositive  bodies,  such  as  the  metals, 
and  gradually  becomes  greater  as  we  pro- 
ceed toward  the  more  electronegative  bodies, 
which,  moreover,  may  even  take  on  new 
negative  electrons.  This  energy  depends 


10   ELECTROLYTIC  IONS  AND  ELECTRONS 

also  upon  the  nature  and  on  the  condition 
of  the  atoms  surrounding  the  one  which 
is  about  to  break  up  into  an  electron  and 
a  positive  ion ;  it  is  extremely  small  for 
bodies  in  aqueous  solution. 

This  being  so,  electrolytic  dissociation,  or 
the  separation  of  a  molecule  into  two  ions,  — 
for  example,  sodium  chloride  into  a  positive 
ion  of  sodium  and  a  negative  ion  of  chlorine, 
—  should  be  considered  to  be  a  consequence 
of  the  dissociation  of  the  metallic  atom. 
This  atom  breaks  up  into  a  positive  ion  of 
sodium  and  into  a  negative  electron,  which 
is  seized  by  the  chlorine  atom,  transforming 
the  latter  into  a  negative  ion.  Once  we 
adopt  this  mode  of  considering  electrolytic 
dissociation,  it,  with  all  its  very  important 
consequences,  enters  into  the  more  general 
electron  theory. 


CHAPTER    II 

THE   ELECTRONS   AND   THE  PHENOMENA 
OF  LIGHT 

WHILE  the  hypothesis  of  the  electrons 
springs  in  such  a  natural  manner  from  elec- 
trolytic phenomena,  it  is  in  an  entirely  differ- 
ent field  of  physics,  namely,  that  of  optics, 
that  it  finds  an  unexpected  and  brilliant  con- 
firmation. 

It  is  a  fact,  now  recognized  by  all,  that 
light  is  a  vibratory  phenomenon,  and  may  no 
longer  be  considered  to  be  due  to  the  emis- 
sion of  discrete  corpuscles  by  luminous 
bodies,  as  Newton  supposed.  In  support  of 
this  many  beautiful  and  classic  experiments 
exist,  with  which  the  names  of  Young,  of 
Fresnel,  and  of  Foucault  are  connected. 
And  when  we  speak  of  light,  we  necessarily 
include  radiant  heat,  because  since  the  cele- 
brated researches  of  Melloni  there  can  exist 
no  doubt  concerning  the  identity  of  the 


12  THE  PHENOMENA  OF   LIGHT 

nature  of  these  phenomena,  which  appear  to 
be  so  different. 

But  the  undulatory  theory  demands  a 
medium  capable  of  propagating  the  waves; 
from  this  arises  the  necessity  of  admitting 
the  existence  of  the  ether ;  that  is,  of  a  sub- 
stance distributed  everywhere  throughout 
interplanetary  and  interstellar  space  and 
throughout  interatomic  space  as  well.  The 
hypothesis  of  the  ether  forces  itself  on  us  in 
an  irresistible  manner,  and  almost  seems  to 
acquire  the  character  of  reality  and  certitude, 
when  we  consider  the  perfection  with  which 
the  undulatory  hypothesis  takes  account  of 
all  optical  phenomena  even  quantitatively 
and  in  the  minutest  detail. 

Following  the  example  of  Fresnel,  light 
vibrations  were  considered  for  a  long  while 
to  be  true  mechanical  vibrations  of  the 
ethereal  and  material  particles,  but  later  it 
was  recognized,  especially  in  consequence  of 
the  work  of  Maxwell,  that  light  waves  could 
be  considered  as  electromagnetic  waves; 
thus  two  distinct  classes  of  physical  phe- 
nomena were  united.  The  electromagnetic 


THE   PHENOMENA  OF   LIGHT  13 

theory  of  light  has  received  in  recent  years 
a  material  support  from  the  well-known  ex- 
periments of  Hertz,  and  later  from  those  of 
other  physicists ;  and  at  present  there  is  per- 
haps no  one  who  will  refuse  to  admit  that 
the  phenomena  of  light  are  in  reality  electro- 
magnetic phenomena,  and  that  light  waves 
differ  from  those  which  Hertz  has  shown 
how  to  produce,  only  in  so  far  as  numerical 
values  are  concerned. 

However,  the  electromagnetic  theory  of 
light,  derived  as  it  is  from  the  properties  of 
the  electromagnetic  field,  does  not  serve  to 
explain  those  phenomena  to  take  account  of 
which  it  was  necessary,  under  the  old  me- 
chanical theory,  to  resort  to  an  action  of  pon- 
derable matter  on  the  ether.  To  complete 
the  theory  accepted  at  present,  it  was  there- 
fore necessary  to  take  account,  in  some 
manner,  of  the  material  atoms;  and  the 
Dutch  physicist,  Lorentz,  had  the  fortunate 
idea  of  considering  the  electric  charges  of 
the  atoms  together  with  the  atoms  them- 
selves. If  merely  the  negative  or  merely 
the  positive  charges  take  part  in  the  light 


14  THE   PHENOMENA   OF   LIGHT 

vibrations,  and  if  both  the  electric  and  mag- 
netic forces  generated  by  their  motion  be 
taken  into  account,  one  arrives  at  an  electro- 
magnetic theory  of  light  capable  of  explain- 
ing even  those  phenomena  which  eluded  the 
theory  based  simply  on  the  formulae  of 
Maxwell  or  of  Hertz. 

Let  us  here  consider  a  most  interesting 
phenomenon  discovered  by  Zeeman,  a  former 
pupil  of  Lorentz,  because  it  is  one  of  those 
by  which  the  relative  independence  of  the 
negative  electrons  and  their  characteristic 
freedom  of  motion  is  most  clearly  demon- 
strated. 

It  is  a  well-known  fact  that  a  luminous 
gas  emits  radiations  of  definite  periods  of 
vibration,  and  not  those  corresponding  to  a 
continuous  series  of  intermediate  periods. 
As  a  result  the  spectrum  of  the  light  emitted 
by  the  gas  reduces  to  a  limited  number  of 
narrow  lines,  which  are  the  images  of  the 
slit  through  which  the  light  is  passed  in 
order  to  analyze  it  with  the  prism.  For  ex- 
ample, the  spectrum  of  the  light  emitted  by 
sodium  in  the  gaseous  state  consists  of  two 


THE  PHENOMENA   OF  LIGHT  15 

yellow  lines  very  near  each  other,  which  with 
low-power  spectroscopes  appear  blended  into 
a  single  line.  Now  Zeeman  showed  that  if 
a  gas  is  placed  in  an  intense  magnetic  field, 
say  between  the  poles  of  a  powerful  electro- 
magnet, each  simple  line  of  its  spectrum  is 
in  general  broken  up  into  a  group  of  new 
lines. 

There  are  two  important  cases  to  be  con- 
sidered :  First,  that  in  which  the  luminous 
ray  which  we  are  considering  is  parallel  to 
the  lines  of  magnetic  force ;  second,  that  in 
which  the  ray  is  perpendicular  to  the  lines  of 
force.  The  general  case  is  naturally  a  trifle 
complicated,  and  for  its  treatment  I  refer  the 
reader  to  original  articles  on  this  subject  (2). 

Let  us  suppose  that  we  have  a  luminous 
gas  between  the  two  opposite  magnetic  poles  ; 
for  example,  vapour  of  cadmium  obtained  by 
passing  electric  sparks  between  two  wires  of 
that  metal.  If  we  examine  the  light  which 
is  propagated  in  the  direction  of  the  lines  of 
force  (Case  I),  that  is,  from  one  pole  toward 
the  other,  we  easily  ascertain  that,  while  the 
green  line  of  the  spectrum  of  cadmium  ap- 


16 


THE   PHENOMENA  OF  LIGHT 


pears  sharp  and  simple  as  in  A  (Fig.  i)  be- 
fore the  magnetic  field  is  set  up,  the  instant 
the  field  is  created  the  line  A  vanishes,  and 
instead  of  it  there  appear  two  new  lines,  B 
and  C,  one  on  either  side  of  the  position  A\ 

A 


B     K    C 


FIG.  i. 

which  the  single  line  at  first  occupied,  and  at 
equal  distances  from  it. 

If  we  examine  with  the  spectroscope  a  ray 
of  light  in  the  equatorial  direction  (Case  II), 
that  is,  perpendicular  to  the  direction  of  the 
magnetic  field,  the  single  line,  A  (Fig.  2),  is 
replaced  by  three  lines,  C,  A',  B,  of  which 
the  one  in  the  middle,  equidistant  from 


THE  PHENOMENA   OF  LIGHT 


the  other  two,  occupies  the  position  of  the 
original  line. 

In  the  case  of  other  spectrum  lines,  either 
the  phenomena  are  identical  with  those 
which  are  exhibited  by  the  green  cadmium 

.A 


FIG.  2. 


line,  or  slightly  more  complicated  effects 
are  obtained.  Thus,  for  example,  of  the 
two  sodium  lines,  the  one  usually  called  /?, 
is  transformed  in  the  second  case  into  four 
lines,  A,  B,  C,  D  (Fig.  3),  while  the  line  Z?2 
changes  into  a  group  of  six  lines  (Fig.  4), 
A,  B,  C,  D,  E,  F. 

Complete  explanation  of  these  phenomena, 


IS 


THE   PHENOMENA   OF   LIGHT 


at  any  rate  in  the  least  complicated  cases,  is 
furnished  by  Lorentz's  theory;  but  for  our 
purpose  it  will  be  sufficient  to  treat  the 
single  case,  to  which  Figure  i  refers,  of  light 
emitted  by  the  vapour  of  cadmium  in  the 
direction  of  the  lines  of  force. 

D, 


A     B 


C     D 


FIG.  3. 

Let  us  consider  an  electrified  particle 
which,  attracted  toward  a  position  of  equi- 
librium, O  (Fig.  5),  vibrates  about  this  point 
with  circular  motion,  describing  a  circumfer- 
ence of  radius  OA.  The  vibrating  electri- 
fied particle  generates  light  waves.  Suppose 
we  study  the  light  which  is  propagated  in 


THE   PHENOMENA  OF   LIGHT 


the  direction  perpendicular  to  the  plane  of 
the  circumference.     If  a  magnetic  force  acts 


A     B     C  .  D     E     F 


FIG.  4. 

in  this  direction,  there  exists  at  every  instant 
an  electromagnetic  force  analogous  to  that 
which  would  act  on  a  short  portion  of  an 
electric  current  coin- 
ciding in  direction  with 
the  velocity  of  the 
particle.  This  force, 
therefore,  will  be  di- 
rected along  the  radius 
OA,  passing  through 
the  moving  particle,  FIG.  5. 


20  THE   PHENOMENA  OF  LIGHT 

and  will  act,  as  the  case  may  be,  either 
from  A  toward  O,  or  in  the  opposite  direc- 
tion. The  effect  of  this  new  force,  which 
augments  or  diminishes  the  force  which 
maintains  the  particle  in  its  orbit,  is  to  vary 
its  vibratory  period,  —  that  is,  the  time  re- 
quired for  the  particle  to  describe  the  cir- 
cumference,—  just  as  a  change  in  the 
intensity  of  the  force  of  gravity  has  the 
effect  of  varying  the  period  of  oscillation 
of  a  pendulum. 

From  the  effect  produced  by  a  magnetic 
field  on  a  circular  vibration  we  may  easily 
pass  to  that  produced  on  any  vibration 
whatsoever  by  a  consideration  like  the  fol- 
lowing :  — 

Light  vibrations  are  perfectly  well  under- 
stood. They  follow  the  same  laws  as  do  the 
small  oscillations  of  a  pendulum,  and  are,  in 
general,  elliptical ;  in  special  cases  they  may 
be  rectilinear  or  circular;  they  are  always 
transverse,  that  is,  they  lie  in  the  plane 
perpendicular  to  the  light  ray.  Now  it  may 
be  shown  that  every  elliptical  vibration  is 
kinematically  equivalent  to  the  resultant  of 


THE   PHENOMENA   OF   LIGHT 


21 


two  circular  vibrations  of  opposite  directions 
of  rotation,  the  one  right-handed  (motion  in 
the  same  sense  as  that 
of  the  hands  of  a 
watch),  the  other  left- 
handed  ;  and  in  ad- 
dition  the  circular 
vibration  which  has  the 
same  direction  of  rota- 
tion as  the  ellipse  EF 
(Fig.  6),  namely  AB,  L' 
has  a  diameter  equal 
to  half  the  sum  of  the  axes  of  the  ellipse, 
while  the  other  circular  vibration,  CD,  has 
a  diameter  equal  to  half  the  difference  of 
the  same  axes.1  If  we  do  not  wish  to  have 

1  Employing  the  usual  symbols,  the  elliptical  vibration, 
referred  to  its  axes,  may  be  represented  by  means  of  its  rec- 
tangular components 


FIG.  6. 


This  is  evidently  equivalent  to  the  resultant  of  two  circular  vi- 
brations, one  of  which  is  right-handed  like  the  given  ellipse, 
and  has  the  components 

a  -^-  o   •    f\  (i  -4-  o        /\ 

x  —      *      sin  \j  v  —      *     cos  v 

2  2 

while  the  other  is  left-handed  and  has  the  components 


22 


THE  PHENOMENA   OF  LIGHT 


recourse  to  a  mathematical  demonstration, 
we  may  convince  ourselves  of  the  proof  of 
this  statement  by  employing  a  special  piece 
of  apparatus  which  serves,  among  other  pur- 


FlG.   7. 

poses,  that  of  effecting  the  composition  of  two 
pendular  vibrations  of  circular  character  (3). 

Two  pendulums  (Fig.  7)  are  suspended 
from  two  fixed  points,  which  for  the  sake  of 
simplicity  are  not  represented  in  the  figure, 


THE   PHENOMENA  OF   LIGHT  23 

placed  one  above  the  other  on  the  same 
vertical  line.  One  of  the  pendulums  con- 
sists simply  of  a  wire  carrying  at  its  lower 
end  a  heavy  ring  and  a  funnel,  A,  filled  with 
sand ;  the  lower  part  of  the  other  consists  of 
a  platform  BC,  situated  below  A,  which  car- 
ries on  one  of  its  sides  a  second  funnel,  D, 
also  filled  with  sand.  The  length  of  the 
first  pendulum  may  be  varied  at  will,  but 
for  the  experiment  with  which  we  have  to 
deal  this  length  should  be  such  that  the 
two  pendulums  have  the  same  period  of 
vibration.  A  simple  electric  device  controls 
the  openings  of  the  funnels  and  thus  pre- 
vents or  allows  the  flow  of  sand. 

At  first  let  us  suppose  the  pendulum  BC 
to  be  fixed  while  we  impart  a  circular  motion 
to  A ;  it  is  easy  to  ascertain  whether  or  not 
this  motion  be  circular  from  the  trace  left  by 
the  sand  on  the  platform  BC.  Let  us  then 
impart  a  circular  motion  to  the  pendulum 
BC  as  well,  but  in  the  direction  opposite  to 
that  of  the  first  pendulum ;  we  may  easily 
ascertain  if  we  succeed  in  doing  this  by 
observing  the  trace  left  by  the  sand  from  the 


24  THE   PHENOMENA   OF   LIGHT 

funnel  D  on  the  plane  of  support.  If,  now 
that  the  two  pendulums  are  vibrating,  we 
allow  the  sand  to  run  out  of  the  funnel  A, 
it  will  form  an  elliptical  trace  on  the  plat- 
form BC.  This  ellipse  becomes  a  straight 
line  if  the  two  component  circular  vibrations 
have  equal  diameters.  We  can  thus  con- 
vince ourselves  of  the  truth  of  the  statement 
made,  and  in  addition  we  learn  that,  when 
the  two  component  circular  vibrations  of  op- 
posite rotation  have  equal  amplitudes,  the 
resulting  vibration  is  rectilinear. 

Returning  now  to  the  case  of  the  particle 
vibrating  in  a  magnetic  field,  it  in  general 
executes  an  elliptical  vibration,  for  which  we 
may  conceive  the  two  equivalent  circular 
vibrations  to  be  substituted.  But  these  last 
are  of  opposite  sign  of  rotation;  if  one  of 
them  is  accelerated  by  the  magnetic  field, 
the  other  must  be  retarded.  As  soon  as 
their  periods  cease  to  be  equal,  they  can  no 
longer  cause  a  single  spectrum  line,  but 
cause  instead  two  new  lines  situated  on 
either  side  of  the  single  primitive  line.  This 
explanation,  furnished  by  Lorentz's  theory 


THE   PHENOMENA  OF   LIGHT  25 

to  explain  the  experiment  of  Zeeman,  was 
proved  correct  by  this  able  experimenter  in 
a  series  of  new  experiments,  which  showed 
that  the  two  new  lines  were  in  fact  due  to 
circular  vibrations,  one  right-handed  and  the 
other  left-handed. 

Suitable  qualitative  and  quantitative  ex- 
periments made  it  possible  to  deduce  two 
very  interesting  results  from  the  Zeeman 
phenomenon.  On  investigating  which  of 
the  two  new  lines  was  due,  for  a  given 
direction  of  the  magnetic  field,  to  the  right- 
handed  vibrations  and  which  to  the  left- 
handed,  the  sign  of  the  charge  of  the 
vibrating  particles  could  be  determined,  and 
it  was  recognized  that,  in  order  to  make  the 
observed  facts  accord  with  their  explanation, 
it  was  necessary  to  admit  that  these  particles 
possessed  a  negative  rather  than  a  positive 
charge.  In  the  second  place,  it  was  possible 
to  obtain  an  approximate  evaluation  of  the 
ratio  of  the  electric  charge  of  the  vibrating 
particle  to  its  mass.  The  result  to  which 
this  led  was,  that  this  ratio  is  more  than  a 
thousand  times  greater  than  that  which  re- 


26  THE  PHENOMENA   OF  LIGHT 

lates  to  the  atom  of  hydrogen  in  electrolysis, 
and  hence  still  greater  than  that  pertaining 
to  the  atoms  of  other  substances. 

This  result  may  be  interpreted  in  several 
ways,  the  more  important  of  which  are  the 
following:  either  the  vibrating  particles  are 
ions,  and  the  charge  of  each  is  more  than 
one  thousand  times  as  great  as  that  which 
pertains  to  each  valency  in  electrolysis;  or 
the  vibrating  particles  have  a  charge  equal 
to  that  of  the  electrolytic  ions,  and  their 
mass  is  less  than  one-thousandth  as  great  as 
that  of  an  ion  of  hydrogen.  The  second 
interpretation  is  naturally  the  one  accepted, 
and  the  vibrating  particles  are  considered  to 
be  free  electrons.  These  therefore  possess, 
or  at  least  there  is  united  with  them,  a  small 
material  mass;  but  we  shall  see  that  this 
same  mass  probably  has  an  electromagnetic 
cause.  At  any  rate,  this  result  is  corrobo- 
rated by  those  which  are  reached  in  other 
ways,  as  will  be  shown  later. 

Lorentz's  theory  receives,  therefore,  a  splen- 
did confirmation  through  the  experiments  of 
Zeeman ;  and  hence  it  may  be  retained,  that 


THE   PHENOMENA   OF   LIGHT  27 

the  structure  of  the  material  atoms  is  such 
as  to  permit  the  negative  electrons,  which 
form  a  part  of  them,  to  vibrate  freely,  while 
the  positive  part  remains  relatively  fixed. 
Thus  we  conceive  a  neutral  atom  to  consist 
of  one  portion  which  in  the  aggregate  has  a 
positive  charge,  and  of  one  or  more  negative 
electrons,  which  move  about  it  like  the  sat- 
ellites about  a  planet,  held  in  their  orbits  by 
the  electric  force. 

The  so-called  oscillators,  or  the  apparatus 
used  to  generate  electromagnetic  waves,  have 
recently  come  into  considerable  prominence. 
One  of  the  possible  but  impractical  forms 
consists  of  an  electrified  body  executing  vi- 
bratory motion ;  for  example,  one  driven  by 
a  sonorous  body  in  vibration.  Now  if  one 
imagines  the  electrified  particle  to  be  replaced 
by  a  simple  electron,  and  if  one  supposes  the 
period  of  vibration  to  be  so  small  that  it 
would  be  expressed  by  a  fraction  whose 
numerator  is  unity  and  whose  denominator 
is  a  number  of  fifteen  places,  then  this 
electron  would  generate  ordinary  light  waves 
instead  of  Hertzian  electromagnetic  waves. 


CHAPTER  III 

NATURE  OF  THE  CATHODE  RAYS  (4) 

THE  phenomena  on  which  we  shall  now 
touch  show  the  negative  electrons  in  the  act 
of  undergoing  very  rapid  motions  of  transla- 
tion instead  of  the  vibratory  motions  which 
we  have  considered  in  the  preceding  chapter. 
Hence  they  present  themselves  under  condi- 
tions favourable  for  a  closer  study,  and  thus, 
by  modifying  their  motion  in  various  ways, 
new  and  interesting  effects  may  be  produced. 
But  for  the  sake  of  clearness  it  will  be  well 
first  to  state  the  principal  characteristics  of 
electric  discharges  in  rarefied  gases. 

Let  us  consider  a  glass  tube,  AC  (Fig.  8), 
through  the  end  walls  of  which  are  fused  two 
platinum  wires  terminating  in  aluminium  elec- 
trodes, A,  C.  If  the  air  pressure  in  the  tube 
is  somewhat  less  than  that  of  the  atmosphere, 
— for  example,  eight  or  ten  millimeters  of 
mercury,  —  and  if  an  electric  discharge  is 

28 


NATURE  OF  THE  CATHODE  RAYS    29 


. 

mm:::':. 


FIG.  8. 


caused  to  pass  from  one  electrode  to  the  other, 
instead  of  the  well-known  loud  and  brilliant 
spark  which  is  formed  in  the  open  air,  a  char- 


30          NATURE   OF   THE   CATHODE   RAYS 

acteristic  luminous  phenomenon  is  obtained, 
in  which  two  regions  are  distinguishable :  the 
positive  luminous  column,  a  sort  of  ill-defined, 
rose-coloured  spark  having  a  blurred  contour 
which  reaches  from  the  anode  up  to  within  a 
short  distance  of  the  cathode ;  and  the  nega- 
tive column,  or  negative  glow,  violet  in  colour 
and  contiguous  with  the  cathode.  Between 
these  two  luminous  regions  there  is  an  inter- 
val called  the  Faraday  dark  space. 

If  now  the  pressure  of  the  air  is  diminished, 
the  luminous  character  changes.  We  will  con- 
cern ourselves  with  the  negative  light  with- 
out considering  further  the  positive  column 
which,  with  increasing  rarefaction,  gradually 
diminishes  both  in  size  and  in  luminous  in- 
tensity, often  subdividing  into  distinct  regions 
separated  by  relatively  dark  intervals  (striated 
discharge).  At  first  the  negative  light  ex- 
tends over  the  entire  cathode,  as  at  C\  in 
case  initially  it  only  covered  the  extremity ; 
but  later,  with  still  further  rarefaction,  it  ex- 
tends all  around  to  greater  and  greater  dis- 
tances, detaching  itself  at  the  same  time  from 
the  electrode,  as  at  C".  In  the  meantime  a 


NATURE   OF  THE   CATHODE   RAYS  31 

new  luminous  stratum  forms  in  contact  with 
the  electrode,  and  thus  the  negative  light  is 
divided  into  two  parts ;  namely,  the  first 
negative  column,  adhering  to  the  cathode, 
and  the  second  negative  column,  or  nega- 
tive glow,  separated  from  each  other  by  a 
relatively  dark  region  which,  to  distinguish 
it  from  the  Faraday  dark  space,  is  called  the 
cathode  dark  space.  Continuing  the  rare- 
faction still  further,  the  two  luminous  nega- 
tive columns  extend  farther  and  farther  out, 
becoming  continually  less  bright  and  sharp 
in  outline  (C'",  Fig.  8).  The  interval  which 
separates  them  also  becomes  greater;  and, 
when  the  highest  rarefaction  is  obtained,  that 
is  to  say  when  the  air  pressure  is  reduced  to 
less  than  one-thousandth  of  a  millimeter  of 
mercury,  almost  every  trace  of  luminosity  in 
the  gas  disappears. 

But  before  this  stage  is  reached  a  new 
phenomenon  appears.  At  first  the  portion 
of  the  walls  of  the  tube  about  the  cathode, 
and  later  that  in  front  of  it,  becomes  lumi- 
nous, diffusing  a  brilliant  light,  usually 
green,  due  to  a  species  of  phosphorescence, 


32    NATURE  OF  THE  CATHODE  RAYS 

or  perhaps  better  of  fluorescence,  since  this  is 
the  name  given  to  the  emission  of  light  by 
fluorspar  and  certain  other  substances,  which 
does  not  last  appreciably  after  the  cause 
which  produced  it  has  ceased.  The  cause 
of  this  phenomenon  is  to  be  sought  in  the 
cathode,  because,  if  an  obstacle  is  placed 
between  it  and  the  wall,  a  very  sharp  shadow 
is  thrown,'  as  if  the  fluorescence  were  excited 
by  invisible  radiations  sent  out  from  the 
cathode.  We  will  now  turn  our  attention 
to  these  radiations,  which  are  called  Cathode 
Rays. 

They  are  propagated  in  straight  lines  and 
leave  the  cathode  in  a  direction  at  right 
angles  to  its  surface ;  hence  if  this  has  the 
form  of  a  concave  mirror,  the  cathode  rays 
converge  practically  to  the  centre  of  curva- 
ture. When  concentrated  in  this  manner 
their  singular  properties  become  more  evi- 
dent, as  Sir  William  Crookes  has  shown  in 
a  very  brilliant  and  suggestive  manner  with 
the  aid  of  cleverly  devised  apparatus. 

The  principal  properties  of  the  cathode 
rays  are  the  following :  they  excite  phospho- 


NATURE  OF  THE  CATHODE  RAYS    33 

rescence  not  only  in  glass,  as  we  have  seen, 
but  in  a  large  number  of  other  bodies,  includ- 
ing those  which  phosphoresce  under  the  ac- 
tion of  light.  The  cathode  rays  heat  bodies 
which  they  strike  and  tend  to  move  them  as 
if  the  impact  were  mechanical.  It  is  pos- 
sible, however,  that  this  mechanical  action 
may  be,  at  least  in  a  large  part,  a  simple  con- 
sequence of  the  preceding  effect.  Finally, 
bodies  struck  by  cathode  rays  become  sources 
of  new  radiations ;  namely,  the  famous  X-rays 
discovered  by  Professor  Rbntgen.  In  order 
to  explain  all  these  phenomena,  Crookes 
brought  forward  his  hypothesis  of  radiant 
matter. 

As  early  as  1816  the  celebrated  Faraday  (5) 
pointed  out  the  possibility  of  a  fourth  state 
of  matter,  as  a  consequence  of  a  hypothetical 
transformation  which  transcends  evaporation 
by  as  much  as  evaporation  transcends  the 
fluid  state ;  or,  he  expressed  his  thought  still 
better  by  saying  that  he  looked  forward  with 
the  greatest  impatience  to  the  discovery  of 
a  new  state  of  the  chemical  elements.  He 
suggested  further,  and  this  has  an  especial 


34    NATURE  OF  THE  CATHODE  RAYS 

interest  with  reference  to  the  theory  with 
which  we  are  now  occupied,  that  the  decom- 
position of  the  metals,  their  recomposition 
and  the  realization  of  the  formerly  absurd 
idea  of  transmutation,  were  problems  which 
chemistry  one  day  must  solve. 

According  to  Crookes,  when  the  electric 
discharge  takes  place  in  a  highly  rarefied 
gas,  very  minute  negatively  electrified  mate- 
rial particles  are  projected  from  the  cathode 
and,  forming  a  fourth  state  of  matter  tran- 
scending the  gaseous  state,  produce  the  ob- 
served effects  as  a  result  of  their  collisions ; 
moreover,  the  trajectories  of  these  particles 
constitute  the  cathode  rays.  It  was  later 
thought  that  the  particles  were  the  actual 
atoms  of  the  gas  residue  which  on  account 
of  its  extreme  rarefaction  presented  such 
new  properties  as  were  brought  to  light  by 
the  rotation  of  the  vanes  of  Crookes'  famous 
radiometer. 

But  some  people,  among  whom  was  the 
illustrious  Hertz,  preferred  to  consider  the 
cathode  rays  as  an  undulatory  phenomenon 
similar  to  light,  having  its  origin  at  the  sur- 


NATURE   OF  THE   CATHODE   RAYS  35 

face  of  the  cathode  and  its  seat  in  the  ether. 
However,  this  opinion  had  very  soon  to  be 
given  up  on  account  of  subsequent  experi- 
ments. Thus,  while  many  physicists,  notably 
J.  J.  Thomson  (6)  in  England,  to  whom  we 
owe  so  much  of  the  present  electron  theory, 
and  Q.  Maiorana  (7)  in  Italy,  were  finding 
out  that  the  velocity  of  the  cathode  rays  is 
noticeably  less  than  that  of  light,  J.  Perrin  (8) 
was  making  it  evident  that  the  cathode 
rays  produce  a  transport  of  negative  electric- 
ity. This  last  effect  may  be  obtained 
even  when  the  rays  have  passed  through  a 
thin  metallic  plate,  as  was  later  shown  by 
Lenard  (9). 

Perrin's  experiment  may  be  made  with  a  dis- 
charge tube  similar  to  that  shown  in  Figure  9. 
The  cathode  consists  of  an  aluminium  disk, 
and  the  anode  ABDE  is  a  cylindrical  box 
with  circular  openings  at  the  centre  of 
the  bases.  This  box  is  in  connection  with 
the  earth,  and  contains  the  conductor  /% 
which  is  connected  to  an  electroscope.  The 
conductor  F  usually  has  the  form  of  a  hol- 
low cylinder  with  an  opening  turned  toward 


36    NATURE  OF  THE  CATHODE  RAYS 

that  in  the  base,  DE,  of  the  anode.  A  nega- 
tive charge  collects  on  the  conductor  /''when 
the  discharge  enters  the  tube.  Evidently 
this  can  only  be  due  to  a  transport  of  charge 
effected  by  the  cathode  rays.  Moreover,  at 
the  approach  of  a  magnet,  the  action  of 
which,  as  we  shall  see,  is  to  make  the  cathode 
rays  assume  a  curved  path,  the  rays  cease  to 


FIG.  9. 

enter  the  cylinder  AD  and  cause  a  luminous 
spot  to  appear  on  the  base  DE,  which  for 
this  purpose  is  usually  coated  with  a  phos- 
phorescent substance.  Now  just  as  soon  as 
the  conductor  F  ceases  to  receive  the  rays, 
it,  in  turn,  ceases  to  receive  electricity. 

Naturally  the  discovery  of  these  facts  fur- 
nished the  strongest  support  for  Crookes' 
theory.  But  countless  recent  experiments 
due  to  many  physicists  have  led  to  a  slight 


NATURE  OF  THE  CATHODE  RAYS    37 

modification,  and  a  better  statement  of  the 
original  hypothesis  and  to  the  admission  that 
the  particles,  which  in  their  rapid  motion 
constitute  the  cathode  rays,  can  be  nothing 
but  the  negative  electrons  themselves.  This 
opinion,  at  present  held  by  all,  rests  princi- 
pally on  the  following  facts  which  have  been 
accurately  verified,  and  with  which  we  shall 
concern  ourselves  in  detail  a  little  later.  In 
the  first  place,  the  cathode  rays  always  have 
identical  properties  whatever  may  be  the 
rarefied  gas  in  which  they  are  formed,  and 
whatever  may  be  the  nature  of  the  cathode ; 
in  the  second  place,  the  moving  negative 
particles  all  possess  that  same  small  mass 
less  than  one-thousandth  that  of  a  hydrogen 
atom  which  is  encountered,  as  we  have  seen, 
in  the  study  of  the  Zeeman  effect,  and  which 
is  deduced  from  the  results  of  various  other 
phenomena  as  well. 

Cathode  rays  may  also  be  produced  with- 
out having  recourse  to  the  electric  discharge. 
Thus,  for  example,  a  body  exposed  to  the 
action  of  light,  or  better  of  ultraviolet  rays, 
emits  electrons.  Unless  the  surrounding 


38    NATURE  OF  THE  CATHODE  RAYS 

gas  is  extremely  rarefied  they  unite  with 
neutral  atoms  and  form  negative  ions;  but 
if  the  gas  is  almost  entirely  removed,  the 
electrons  remain  free,  and  on  leaving  the 
body  form  true  cathode  rays  (10),  which  in 
general  possess  a  velocity  less  than  that 
which  they  have  in  discharge  tubes.  This 
velocity  decreases  as  the  negative  potential 
of  the  illuminated  body  is  diminished. 

Also  in  the  case  of  the  cathode  rays  the 
mass  of  the  electron  was  not  separately  de- 
termined, but  rather  the  ratio  between  the 
electric  charge  of  any  electron  and  its  mass. 
Such  a  determination  as  this  is  based  on  the 
effects  produced  on  the  cathode  rays  by  elec- 
tric or  magnetic  forces,  and  these  effects  are 
in  good  accord  with  the  accepted  hypothesis. 
In  fact,  it  is  clear  that  when  an  electric  force 
acts  on  negative  particles  in  motion,  they 
should  deviate  from  their  ordinary  rectilinear 
path ;  and  since  an  electrified  particle  in 
motion  should  behave  in  a  manner  analo- 
gous to  a  current,  or  more  accurately  to  an 
element  of  current,  it  follows  that  the  par- 
ticle itself  should  deviate  from  its  ordinary 


NATURE  OF  THE  CATHODE  RAYS    39 

path  when  it  is  exposed  to  the  action  of  a 
magnetic  field.  But  we  shall  consider  such 
phenomena  and  the  measurements  relating 
to  them  a  little  farther  on. 


CHAPTER   IV 

THE   IONS   IN   GASES  AND   IN   SOLIDS 

IN  electrolytes  the  electrons  are  joined  to 
the  neutral  atoms  to  form  free  ions,  and  the 
motion  of  these  ions  is  what  constitutes  the 
electric  current.  At  present  the  opinion  is 
held  that  the  same  thing  happens  in  gases ; 
namely,  that  when  a  gas  possesses  electric 
conductivity,  it  owes  it  to  the  presence  of 
ions,  and  to  their  motion  under  the  action 
of  electric  forces.  The  hypothesis  of  the 
ionization  of  gases,  which,  for  a  long  time, 
was  held  by  very  few,  is  now  generally 
admitted  in  consequence  of  the  numerous 
experiments  made  in  recent  years. 

We  are,  then,  of  the  opinion  that  a  gas 
contains  free  ions.  These  are  ordinarily 
present  in  such  a  small  number  that  the 
resulting  conductivity  is  very  small.  But 
there  are  circumstances  in  which,  by  the 

40 


THE   IONS   IN   GASES  AND   IN   SOLIDS       41 

action  of  appropriate  external  energy,  the 
gas  is  ionized ;  that  is  to  say,  many  of  its 
atoms  are  broken  up  into  positive  ions  and 
negative  electrons.  If  the  gas  is  not  suf- 
ficiently rarefied,  the  electrons  unite  with 
the  neutral  atoms  and  form  negative  ions. 
Moreover,  certain  facts  seem  to  indicate  that 
atoms  or  neutral  molecules  can  unite  with 
ions  to  form  groups  which,  while  having  the 
usual  charge  of  the  ions,  possess  masses 
much  greater  than  those  which  can  pertain 
to  a  simple  ion. 

The  most  natural  explanation  of  the 
known  facts,  and  in  particular  of  those 
about  to  be  mentioned,  is  that  the  electrical 
conductivity  of  gases  is  due  to  the  presence 
of  electrified  particles  which  are  free  to  move 
between  its  molecules. 

An  ionized  gas  loses  its  conductivity 
when  passed  through  minute  interstices,  say 
through  a  mass  of  glass  wool,  or  through 
long  and  fine  metallic  tubes,  or  is  made  to 
bubble  through  a  conducting  liquid  (n), 
which,  however,  should  not  contain  any 
radio-active  substance.  The  same  result  is 


42       THE   IONS   IN   GASES  AND   IN   SOLIDS 

obtained  if  the  gas  is  made  to  pass  between 
two  oppositely  electrified  conductors  in  such 
a  way  that  it  may  serve  as  a  conductor  for 
the  current.  In  the  first  case,  the  phenome- 
non is  explained  by  the  attraction  exerted  on 
the  ions  by  the  bodies  near  which  they  pass ; 
in  the  second  case,  the  two  conductors 
attract  and  hold  the  ions  which  carry  a 
charge  opposite  in  sign  to  their  own  and  so 
remove  them  from  the  gas. 

The  manner  in  which  an  ionized  gas  be- 
haves when  it  is  carrying  an  electric  cur- 
rent is  also  in  perfect  accord  with  the 
accepted  hypothesis.  Let  us  suppose  that 
we  have,  for  example,  two  parallel  metallic 
disks,  one  of  which  communicates  with  the 
insulated  pole  of  a  battery,  and  the  other 
with  an  electrometer.  If  we  ionize  the  air 
between  the  disks  by  passing  Rontgen  rays 
through  it,  and  if  we  vary  the  value  of  the 
potential  furnished  by  the  battery,  we  find 
that  the  gas  fails  to  follow  the  well-known 
law  of  Ohm,  which  holds  for  constant  elec- 
tric currents,  and  according  to  which  the 
intensity  of  the  current  in  a  conductor  in- 


THE   IONS  IN   GASES  AND   IN   SOLIDS       43 

creases  in  proportion  to  the  difference  of 
potential  between  its  ends.  In  fact,  the 
intensity  of  the  current  measured  by  the 
charge,  which  is  acquired  in  a  given  time  by 
the  disk  in  communication  with  the  elec- 
trometer, increases  considerably  less  rapidly 
than  the  potential.  The  intensity  even 
finally  assumes  a  limiting  value  which  does 
not  increase  as  the  potential  of  the  battery  is 
raised.  When  the  current  has  attained  this 
value,  called  the  saturation  value,  all  the 
ions  generated  in  a  given  time  by  the  Rbnt- 
gen  rays  (or  in  general  produced  by  what- 
ever source  of  ionization  is  employed)  are 
utilized  in  transmitting  the  current  in  this 
same  time.  An  increase  in  potential  is  of 
no  effect,  as  there  are  not  a  greater  number 
of  ions  to  be  disposed  of. 

Moreover,  a  curious  phenomenon  met 
with  by  the  writer  (12),  and  which  was 
confirmed  and  rightly  interpreted  by  J.  J. 
Thomson  and  E.  Rutherford,  is  obviously 
explained  by  the  accepted  theory.  The  phe- 
nomenon is  the  following:  if  the  distance 
between  the  two  metallic  disks  considered 


44      THE   IONS   IN   GASES  AND   IN   SOLIDS 

above  is  varied,  the  intensity  of  the  current 
which  traverses  the  ionized  air  between  them 
varies  as  well,  but  in  a  manner  contrary  to 
that  which  one  would  suppose.  In  fact,  the 
intensity  of  the  current  increases,  within 
certain  limits,  with  an  increase  of  the  dis- 
tance. This  is  easily  explained  when  we 
reflect  that  with  an  increase  in  the  distance 
between  the  plates  there  is  an  increase  in 
the  amount  of  air  in  which  the  phenomenon 
takes  place,  and  in  consequence  also  in  the 
number  of  ions,  which,  by  their  motion,  con- 
stitute the  saturation  current. 

The  ions  in  gases  move  around  between 
the  molecules,  frequently  colliding  with 
them.  New  ions  may  form  by  the  breaking 
up  of  neutral  molecules,  and  ions  of  opposite 
sign  may  recombine  into  molecules.  This 
last  action,  namely,  the  disappearance  of 
ions,  is  continually  taking  place,  and  it  is 
because  of  this  that  the  number  of  ions  does 
not  increase  beyond  a  certain  limit  under 
the  action  of  an  ionizing  cause. 

If  the  ions  are  generated  in  a  single  region 
of  the  gas,  they  diffuse  into  the  remaining 


THE   IONS   IN   GASES  AND   IN   SOLIDS       45 

portion.  In  gases  under  ordinary  pressure 
the  velocity  of  diffusion  is  usually  extremely 
small  because  of  frequent  collisions;  but  if 
an  electric  field  acts,  the  velocity  of  diffusion 
becomes  large ;  the  first  time  that  a  measure- 
ment of  this  kind  was  made  (13)  the  velocity 
was  found  to  be  several  decimeters  per 
second. 

Ultraviolet  rays,  cathode  rays,  Rbntgen 
rays,  rays  emitted  by  radio-active  substances, 
heating  to  a  relatively  high  temperature,  are 
all  causes  of  ionization.  This  is  greater  or 
less  according  to  circumstances,  and  is  limited, 
as  has  already  been  mentioned,  by  a  continual 
recomposition  of  atoms  and  neutral  mole- 
cules. 

But  there  exists  still  another  cause  of 
ionization,  to  which  in  reality  some  of  the 
above  causes  reduce ;  this  is  the  collision  of 
the  ions  (and,  in  fact,  of  the  electrons  as  well, 
since  probably  some  of  these  exist,  at  least 
momentarily,  in  the  free  state  in  gas  under 
ordinary  pressure)  with  the  atoms  and  mole- 
cules. When  an  ion  possesses  a  sufficiently 
high  velocity,  it  can  furnish  the  energy  neces- 


46      THE   IONS   IN   GASES  AND   IN   SOLIDS 

sary  to  transform  an  atom  into  a  positive 
ion  and  a  negative  electron,  and  hence  also 
to  transform  a  molecule  into  two  ions  of 
opposite  sign.  Let  us  briefly  take  up  these 
various  means  of  ionizing  a  gas. 

Light  radiations,  and  especially  the  ultra- 
violet, may  ionize  a  gas  in  two  different 
ways.  If  they  strike  a  solid  or  liquid  body, 
they  produce  an  emission  of  negative  elec- 
trons which  results  in  the  rapid  discharge  of 
the  body,  if  it  was  negatively  electrified,  and 
even  the  formation  on  it  of  a  positive  charge, 
as  has  been  demonstrated  by  the  writer  (14). 
Ordinarily  the  experiment  is  performed  with 
metals,  because  the  effect  is  rather  weak 
with  liquids,  and  solid  insulators  are  not  so 
well  adapted  to  quantitative  determinations. 
As  a  source  of  active  radiations,  the  invisible 
ultraviolet  rays  emitted  by  an  arc  light  or 
by  an  electric  spark  are  employed,  although 
certain  bodies,  such  as  the  alkaline  metals 
and  amalgamated  zinc,  give  a  marked  effect 
even  with  visible  radiations.  Now  if  the 
electric  field  determined  by  the  negative 
charge  of  the  body  is  sufficiently  intense, 


THE   IONS   IN   GASES   AND   IN   SOLIDS      47 

the  negative  electrons  which  are  emitted  may 
acquire  a  velocity  great  enough  to  ionize  the 
neutral  atoms  by  impact. 

But  even  directly  the  more  refrangible 
ultraviolet  radiations  given  off  by  the  electric 
spark  cause  ionization  of  the  gas  through 
which  they  pass,  as  was  demonstrated  by 
Lenard  (15),  who  allowed  the  radiations  from 
a  spark  formed  between  aluminium  wires 
to  fall  on  electrified  bodies.  These  became 
discharged  with  about  the  same  rapidity 
whether  they  were  charged  positively  or 
negatively  and  whatever  was  the  nature  and 
condition  of  their  surfaces.  All  this  could 
not  be  attributed  to  a  surface  action,  but 
rather  to  an  action  on  the  mass  of  the  air 
traversed  by  the  radiations;  that  is,  to  the 
ionization  which  they  produced.  An  experi- 
ment which  may  be  repeated  also  with  other 
sources  of  ionization  confirmed  this  explana- 
tion. It  consists  in  blowing  the  air  from  the 
place  where  it  is  ionized  to  another  place, 
where,  as  a  result  of  the  conductivity  which 
it  retains  for  a  certain  time,  it  brings  about 
the  discharge  of  electrified  bodies.  In  order 


48       THE   IONS   IN   GASES  AND    IN   SOLIDS 

to  cause  the  effect  to  cease  it  is  sufficient 
to  intercept  the  radiations. 

It  appears  that  only  the  more  rapid  ultra- 
violet vibrations  produce  direct  ionization 
of  gases  in  any  marked  degree.  Indeed,  the 
experiments  described  above  do  not  succeed 
except  when  the  path  traversed  in  the  air 
by  the  radiations  is  reduced  to  but  a  few 
centimeters,  and  it  is  a  well-known  fact  that 
the  most  refrangible  ultraviolet  radiations 
are  very  rapidly  absorbed  by  air  at  ordinary 
pressure. 

The  cathode  rays  which  are,  as  we  have 
seen,  nothing  but  negative  electrons  in 
motion,  ionize  a  gas,  as  will  be  explained 
before  long  in  some  detail. 

With  regard  to  the  Rontgen  rays,  which 
apparently  are  the  manifestation  of  ether 
waves  generated  by  the  sudden  variations 
in  velocity  of  the  electrons,  the  ionization 
of  gases  produced  by  them  seems  to  be  due 
to  a  sudden  electric  impulse  produced  in  the 
electrons  of  the  gaseous  atoms. 

Finally,  a  rise  in  temperature,  which  is 
equivalent  to  an  increase  in  atomic  velocity 


THE   IONS   IN  GASES   AND   IN   SOLIDS       49 

and  apparently  to  an  increase  in  the  velocity 
of  the  negative  electrons  as  well,  naturally 
tends  to  free  the  latter  from  their  bond  with 
the  positive  part  of  the  atom.  A  red-hot 
metallic  wire  ionizes  the  gas  in  contact  with 
it,  and  gases  in  flames  always  appear  to  be 
strongly  ionized. 

In  order  that  the  molecules  of  the  gas  may 
become  ionized  by  the  impact  of  those  ions 
which  already  exist  in  it,  it  is  generally  neces- 
sary to  expose  the  gas  to  the  action  of  suffi- 
ciently intense  electrical  forces.  In  too 
weak  a  field,  the  ions,  while  they  follow  the 
electric  force,  do  not  acquire  a  sufficient 
velocity  between  one  collision  and  the  next, 
and  the  effect  of  the  collisions  is  to  keep  this 
velocity  constantly  at  a  low  value,  since  natu- 
rally a  part  of  the  energy  of  motion  of  the 
ions  is  given  up  to  the  molecules  which  have 
been  struck.  Under  such  circumstances  the 
paths  described  by  the  ions  cannot  differ 
much  from  the  lines  of  electric  force,  or,  in 
other  words,  the  ions  must  continually  move 
very  nearly  in  the  direction  of  the  force  which 
urges  them.  The  so-called  phenomena  of 


50      THE   IONS   IN   GASES   AND   IN   SOLIDS 

the  electric  shadow  and  similar  ones  of  which 
the  reader  will  find  a  partial  treatment  in 
another  place  (16),  are  the  immediate  conse- 
quence of  this. 

But  if  a  sufficiently  strong  electric  field 
acts  on  the  gas,  ionization  by  impact  takes 
place,  and  it  is  on  this  phenomenon  that  a 
satisfactory  explanation  of  the  complex  and 
varied  phenomena  of  the  electric  discharge  is 
at  present  founded.  In  order  to  take  this 
matter  up,  we  would  be  obliged  to  go  beyond 
the  limits  assigned  to  this  chapter;  but  in 
view  of  that  which  follows,  it  will  be  useful 
to  call  attention  by  way  of  an  example,  to 
the  explanation  which  is  given  of  the  forma- 
tion of  the  two  columns  of  negative  light  and 
the  dark  space  included  between  them  in  the 
case  of  the  electric  discharge  in  highly  rare- 
fied gases. 

The  phenomenon  is  started  by  the  few 
electrons  existing  in  the  gas,  or  perhaps  also 
by  the  negative  electrons  expelled  from  the 
cathode.  These  electrons  move  with  accel- 
erated motion  and  rapidly  acquire  a  velocity 
sufficient  to  make  them  capable  of  ionizing 


THE   IONS  IN   GASES  AND   IN   SOLIDS       51 

by  impact  the  gas  molecules  at  some  distance 
from  the  cathode.  This  gives  rise  to  the 
second  negative  column,  or  negative  glow, 
which  is  thus  a  region  of  the  gas  where  ion- 
ization  takes  place.  The  electric  force  drives 
the  positive  ions  created  in  this  manner 
toward  the  cathode,  close  to  which  they 
possess  the  velocity  required  to  ionize  the 
gas  molecules.  This  causes  the  formation 
of  the  first  column  of  negative  light. 

The  electrons  produced  in  this  region 
move  away  from  the  cathode,  and  in  this  way 
the  two  regions  of  ionization  furnish  each 
other  the  necessary  ions  or  electrons.  The 
cathode  dark  space  is  thus  simply  the  region 
traversed  by  the  electrons  constituting  the 
cathode  rays,  and  especially  by  the  positive 
ions  which  move  toward  the  cathode,  before 
they  have  acquired  the  velocity  necessary  to 
produce  ionization. 

We  will  not  concern  ourselves  further 
with  the  negative  electrons  after  they  have 
arrived  at  the  second  negative  column  ;  how- 
ever, it  is  of  interest  to  us  to  know  what  be- 
comes of  the  positive  ions  aftet  they  arrive 


52       THE   IONS   IN   GASES   AND   IN   SOLIDS 

at  the  cathode.  Some  of  them  naturally 
become  neutralized  by  the  negative  elec- 
trons ;  but  others,  on  account  of  their  velocity, 
which  varies  in  direction  as  a  result  of  colli- 
sions, may  bend  around  the  cathode,  or  even 
pass  through  it,  if  it  is  provided  with  open- 
ings or  canals,  or  if  it  is  made  of  wire  gauze. 
Beyond  the  cathode  the  positive  ions  then 
constitute  the  positive  or  anode  rays,  analo- 
gous to  the  cathode  rays,  and  which  are  often 
called  canal  rays,  the  name  given  them  by 
Goldstein. 

An  electric  or  a  magnetic  field  causes  a 
deviation  of  the  positive  rays,  but  in  the 
direction  opposite  to  that  which  would  be 
observed  with  cathode  rays;  it  is  precisely 
on  account  of  this  that  the  conclusion  is 
reached,  that  these  rays  consist  of  positively 
electrified  particles  in  motion.  However,  the 
deviation  is  noticeably  smaller  for  the  posi- 
tive rays  than  for  the  cathode  rays  under 
similar  conditions.  From  measurements  of 
the  deviation  it  may  be  shown  that  the  par- 
ticles in  motion  do  not  possess  a  minute  mass 
as  in  the  case  of  the  cathode  rays,  but  a  mass 


THE   IONS   IN   GASES  AND   IN   SOLIDS       53 

comparable  with  that  of  atoms  or  of  electro- 
lytic ions.  In  this  case,  therefore,  we  have 
to  deal  not  with  positive  electrons,  but  with 
ions  and  probably  with  groups  of  greater 
mass  as  well. 

If  we  admit  electric  conductivity  to  be  a 
phenomenon  of  convection  in  gases  as  well 
as  in  liquids,  the  analogous  hypothesis  for 
solid  conductors,  already  stated  in  Chapter  I, 
becomes  all  the  more  natural.  And,  inas- 
much as  it  appears  that  only  the  negative 
electrons,  and  not  the  positive,  can  exist  iso- 
lated, it  is  held  that  the  electric  current  in  a 
conductor  consists,  at  least  principally,  in  the 
motion  of  negative  electrons.  The  experi- 
mental evidence  that  metals  do  not  offer  an 
insurmountable  obstacle  to  the  motion  of  the 
electrons  is  furnished  by  their  perviousness 
to  the  cathode  rays.  Without  entering  into 
details  we  may  add  that  this  mode  of  consid- 
ering the  current  permits  us  to  explain  fairly 
well  various  observed  facts,  as,  for  example, 
the  proportionality  between  the  thermal  and 
electric  conductivity  of  various  bodies,  and  to 
explain  such  phenomena  as  those  relative  to 


54      THE   IONS   IN   GASES   AND   IN   SOLIDS 

the  optical  properties  of  metals.  Thus  the 
electron  theory  not  only  finds  no  contradic- 
tion in  phenomena  of  this  sort,  but  also 
shows  itself  able  to  furnish  a  simple  explana- 
tion of  them. 


CHAPTER   V 

RADIO-ACTIVITY 

THE  discovery  of  the  so-called  X-rays  by 
Professor  Rontgen  early  in  1896  gave  rise  to 
numerous  experiments  relating  to  the  possi- 
ble existence  of  other  radiations  capable  of 
affecting  photographic  films  and  of  passing 
through  opaque  bodies.  Effects  of  this  sort 
were  described  by  Le  Bon;  but  we  will  not 
consider  these,  as  it  was  recognized  that  the 
effects  attributed  by  this  experimenter  to  a 
new  radiation  which  he  called  "  black  light " 
proceeded,  at  least  in  nearly  every  case,  from 
causes  which  have  no  intimate  relation  with 
the  subject  which  we  are  now  about  to  treat. 

Certain  experiments  which  have,  on  the 
other  hand,  a  relationship  with  radio-activity 
were  made  by  Henry  (17)  with  phosphores- 
cent sulphide  of  zinc ;  by  Niewenglowski  (18) 
with  calcium  sulphide  ;  and  by  Becquerel  (19) 

55 


56  RADIO-ACTIVITY 

with  double  sulphate  of  uranium  and  potas- 
sium. The  result  of  these  experiments  is 
that  these  substances  emit  rays  capable  of 
passing  through  opaque  bodies  and  acting 
on  a  photographic  film,  when  phosphores- 
cence is  excited  in  them  by  exposure  to 
light  or  to  X-rays. 

When  Becquerel  began  his  experimenta- 
tion, he  had  a  special  object  in  view.  It  was 
already  known  that  the  X-rays  had  their 
origin  at  that  part  of  the  wall  which  is  ren- 
dered luminescent  where  the  cathode  rays 
strike ;  it  was  consequently  natural  to  sup- 
pose that  phosphorescence  and  the  emission 
of  X-rays  were  related  phenomena,  though 
later  it  was  recognized  that  such  is  not  the 
case.  H.  Becquerel  therefore  wished  to  de- 
termine whether  bodies  rendered  phospho- 
rescent by  the  action,  not  of  cathode  rays, 
but  of  light,  would  emit  X-rays.  And  since 
these  rays  can  affect  a  photographic  film 
even  when  it  is  surrounded  by  opaque  ob- 
jects, the  French  physicist  placed  various 
bodies  above  a  film,  protected  in  this  man- 
ner from  the  light,  and  exposed  the  whole 


RADIO-ACTIVITY  57 

to  sunlight.  After  several  unsuccessful  at- 
tempts, he  obtained  a  marked  effect  with 
crystalline  laminae  of  the  double  sulphate  of 
uranium  and  potassium,  since  on  developing 
the  film  he  saw  an  image  of  the  laminae  and 
the  shadow  of  a  silver  coin  which  had  been 
placed  below  one  of  them.  It  seemed,  there- 
fore, that  the  phenomenon  he  had  predicted 
had  really  taken  place.  But  Becquerel  hap- 
pened also  to  obtain  the  same  effect  with 
poor  illumination  on  a  cloudy  day,  from 
which  he  suspected  that  the  effect  itself  did 
not  depend  on  the  action  of  the  light.  And, 
in  fact,  he  very  soon  proved  (20)  that  the 
uranium  salt  continually  and  spontaneously 
emitted  rays  able  to  go  through  opaque 
bodies  and  to  act  on  photographic  films 
without  the  necessity  of  its  being  exposed 
to  the  light. 

Further  research  showed  (21)  that  the  rays 
from  the  uranium  salt  share  with  the  X-rays, 
not  only  the  property  of  traversing  opaque 
bodies,  of  acting  on  photographic  films,  of 
rendering  phosphorescent  bodies  luminous, 
and,  as  was  later  shown,  the  negative  prop- 


58  RADIO-ACTIVITY 

erty  of  not  being  susceptible  of  reflection 
and  refraction,  and  hence  of  polarization,  but 
also  another  property  which  was  recognized 
shortly  after  the  X-rays  were  discovered  (22); 
namely,  that  of  ionizing  gases  through 
which  they  pass.  A  method  of  studying 
the  Becquerel  rays,  which  is  more  rapid 
than  the  photographic  method,  is  based  on 
this  property;  it  consists  in  the  measure- 
ment of  the  velocity  with  which  an  electri- 
fied body  becomes  discharged  when  the 
surrounding  gas  is  exposed  to  the  action  of 
the  rays. 

For  this  purpose  we  may  make  use  of  any 
electrometer  connected  to  a  metallic  disk 
placed  at  a  short  distance  from  a  second  disk ; 
and  we  may  experiment  in  two  ways.  Either 
the  second  disk  is  put  in  connection  with  the 
earth  and  then  one  notes  the  rate  of  diminu- 
tion of  an  electric  charge  communicated  to 
the  electrometer  when  the  air  between  the 
two  disks  is  ionized  by  the  radio-active  body ; 
or  the  second  disk  is  charged  and  the  rapidity 
with  which  the  electrometer  deviates  is  ob- 
served. With  very  active  bodies  a  galvanom- 


RADIO-ACTIVITY  59 

eter  may  be  employed,  but  if  bodies  of  weak 
radio-activity  are  to  be  investigated,  it  is 
better  to  use  a  gold-leaf  electrometer,  or  bet- 
ter still,  an  electroscope,  with  but  a  single 
leaf  consisting  simply  of  a  vertical  metallic 
rod,  AB  (Fig.  10),  at  the  upper  end  of  which 
a  very  light  gold  or  aluminium  leaf, 
CD,  is  fastened.  In  order  to  secure 
good  insulation  the  rod  is  supported 
by  a  small  piece  of  sulphur,  S.  The 
electrical  capacity  of  the  conduct- 
ing system  ABCD  is  extremely 
small,  and  hence  it  results  that  the 
diminution  in  the  divergence  of  the 
leaf  CD  is  not  too  slow.  The  elec- 
troscope becomes  an  electrometer,  'IG' Ia 
if  the  position  of  CD  is  observed  by  means 
of  a  microscope  and  ocular  scale.  The 
potential  corresponding  to  each  division  of 
the  scale  may  be  determined  in  advance  by 
the  use  of  a  battery  of  small  accumulators. 
The  writer  prefers  a  form  of  electrometer 
slightly  different  from  that  which  has  become 
classic  for  the  study  of  feebly  radio-active 
bodies.  The  sulphur  or  fused  quartz  insu- 


6o 


RADIO-ACTIVITY 


lator  61  (Fig.  n)  is  quite  slender  and  held 
with  mastic  or  gutta-percha  to  the  bottom  of 
a  small  bell-shaped  piece  of  metal  connected 
with  the  rod  AB.  This  form  of  support 
obviates  or  reduces  the  creeping  of  the 
charge  from  the  rod  to  the  surface  of  the 
insulator.  Moreover,  the  writer  in 
again  adopting  an  arrangement 
which  he  devised  many  years  ago, 
uses  an  ordinary  millimeter  scale 
placed  at  several  meters'  distance 
from  the  electrometer,  instead  of 
one  annexed  to  the  ocular.  A  con- 
verging acromatic  lens  forms  a  real 
FIG.  ii.  image  of  the  scale  in  the  plane  in 
which  the  metallic  leaf  moves,  and  thus  both 
the  leaf  and  the  scale  are  seen  simultaneously 
in  the  field  of  the  microscope. 

In  one  of  the  electrometers  lately  con- 
structed by  the  writer  the  rod  and  the  gold 
leaf  have  scarcely  one-fourth  the  dimensions 
shown  in  Figure  n.  It  is  particularly  well 
adapted  to  the  demonstration  of  radio-activity, 
since  it  is  so  sensitive  that,  when  one  of  the 
uranium  salts  is  approached  in  such  a  way 


RADIO-ACTIVITY  6 1 

that  the  rays  emitted  by  it  pass  through  the 
thin  aluminium  walls  of  the  electrometer 
case,  the  gold  leaf,  when  previously  electri- 
fied, may  even  with  the  unaided  eye  be  seen 
to  droop. 

As  more  and  more  experiments  were 
made,  it  was  established  that  all  the  com- 
pounds of  uranium  are  radio-active,  that  is, 
they  emit  Becquerel  rays,  and  their  activity 
is  in  proportion  to  the  quantity  of  uranium 
which  they  contain;  this  shows  that  radio- 
activity is  a  property  of  the  uranium  atom 
and  one  which  remains  unaltered  when  the 
atom  itself  enters  into  combination  with  the 
atoms  of  other  chemical  elements. 

Thorium,  as  well  as  uranium,  is  radio- 
active, to  a  slightly  different  degree,  as  was 
independently  established  by  Schmidt  (23) 
and  by  Mme.  Curie  (24). 

If  it  had  not  been  for  the  discovery  of  cer- 
tain bodies  whose  radio-activity  is  hundreds 
and  even  thousands  of  times  greater  than 
that  of  uranium,  it  would  have  been  perhaps 
very  difficult  to  have  studied  radio-activity  in 
a  thorough  manner  and  to  have  determined 


62  RADIO-ACTIVITY 

the  peculiarities  and  the  probable  original 
cause  of  this  interesting  phenomenon,  or,  at 
least,  all  this  would  have  required  much 
time  and  most  accurate  research. 

M.  and  Mme.  Curie  chanced  to  find  cer- 
tain specimens  of  chalcolite  and  pitchblende 
(in  particular  that  which  is  mined  at  Joachims- 
thai),  which  were  somewhat  more  active  than 
pure  uranium.  Recalling  that  radio-activity 
is  an  atomic  property,  the  phenomenon  could 
not  be  attributed  to  uranium  contained  in 
these  minerals,  and  it  was  necessary  to  sup- 
pose that  an  unknown  substance,  more  active 
than  uranium  itself,  was  present.  Having 
recourse  to  physical  and  chemical  methods 
of  separation,  certain  compounds  of  bismuth 
were  extracted  from  these  minerals  having  a 
radio-activity  as  much  as  four  hundred  times 
that  of  uranium  (25).  The  name  polonium 
was  given  to  the  unknown  substance  con- 
tained in  these  compounds,  the  radio-activity 
of  which  diminishes  slowly  as  time  goes  on. 
Later  the  Curies  and  M.  Bemont  (26)  extracted 
from  pitchblende  a  small  quantity  of  a  very 
active  body,  chemically  analogous  to  barium 


RADIO-ACTIVITY  63 

and  associated  with  it  in  almost  every  re- 
action, to  which  they  gave  the  name  radium. 
Another  radio-active  body  associated  with 
thorium  and  possessing  similar  chemical  prop- 
erties was  discovered  by  M.  Debierne  (27), 
who  called  it  actinium. 

Various  other  investigators  have  suc- 
ceeded in  deriving  still  other  noticeably 
radio-active  substances  from  many  different 
minerals,  but  especially  from  pitchblende; 
the  nature  of  these  substances  is  not  yet  well 
understood.  In  particular  Elster  and  Gei- 
tel  (28)  obtained  a  rather  radio-active  sulphate 
of  lead  which  seems  to  contain  simply  radif- 
erous  barium.  Nor  is  the  radio-active  lead 
obtained  by  Giesel  (29)  any  better  character- 
ized, while  that  found  by  Hofmann  and 
Strauss  (30)  would,  in  some  respects,  seem 
to  resemble  polonium.  The  active  lead,  or 
radio-lead  of  these  authors,  exhibited  one 
property  worthy  of  mention.  Under  certain 
conditions  the  active  sulphate  of  lead  loses  a 
large  part  of  its  radio-activity,  which  it  later 
slowly  recovers ;  but  if  it  is  exposed  to  the 
bombardment  of  the  cathode  rays,  its  radio- 


64  RADIO-ACTIVITY 

active  properties  are  fully  restored  in  a  few 
minutes. 

Recently  Hofmann  and  Wblfl  (31)  have 
partially  separated  the  active  substance  from 
the  inactive  lead,  and  have  determined  that 
it  differs  from  the  polonium  of  Mme.  Curie 
in  the  constancy  of  its  radio-activity. 

The  substance  called  radio- tellurium  by 
Marckwald  (32)  also  resembles  polonium  in 
its  properties,  the  only  difference  being  that 
its  radio-activity  does  not  seem  to  diminish 
with  time.  It  would  seem  that  radio-tellu- 
rium is  the  body  to  which  the  activity  of 
the  bismuth  extracted  from  the  Joachimsthal 
pitchblende  is  due,  and  it  is  considered  by 
this  author  to  be  a  new  body  belonging  to 
the  sulphur  and  tellurium  series,  because  it  is 
deposited  on  sticks  of  bismuth  or  antimony 
introduced  into  an  acid  solution  of  active 
chloride  of  bismuth.  Marckwald  obtained 
in  this  manner  about  six  decigrams  of  quite 
active  material  from  850  grams  of  the  salt. 
An  active  bismuth  quite  similar  to  that  of 
the  other  investigators  was  discovered  by 
Giesel  (33),  who  later  separated  from  it  a  prod- 


RADIO-ACTIVITY  65 

uct  identical  with  that  obtained  by  Marck- 
wald.  Finally  a  supposed  new  radio-active 
element  was  separated  from  thorium  by 
Baskerville,  who  called  it  carolinium  (34). 

As  we  see,  a  great  uncertainty  still  exists 
regarding  the  nature  and  even  the  separate 
existence  of  the  greater  part  of  the  radio- 
active substances;  so  much  so,  in  fact,  that 
there  are  some  who  consider  the  effects  at- 
tributed to  Mme.  Curie's  polonium  as  due  to 
an  induced  radio-activity^  that  is,  to  a  transi- 
tory phenomenon,  of  which  we  shall  speak 
later;  there  are  others  who  believe  radio- 
active thorium  and  actinium  to  be  identical. 
The  chemistry  of  the  radio-active  bodies  is 
only  in  its  initial  stages,  and  at  present 
radium,  whose  characteristic  spectrum  is  now 
known,  is  the  only  substance  the  existence 
of  which,  as  an  element  distinct  from  the 
others,  seems  to  be  sure. 

Radium  has  never  been  prepared  in  the 
free  state,  but,  nevertheless,  several  of  its  salts 
are  known.  For  example,  the  small  quan- 
tity of  radio-active  substance,  which  the 
Curies  were  first  able  patiently  to  obtain  from 


66  RADIO-ACTIVITY 

several  tons  of  pitchblende  residues,  remain- 
ing after  the  uranium  had  been  extracted, 
was  the  chloride  of  radium.  These  residues 
contained  compounds  of  almost  all  the  met- 
als, among  which  were  barium,  bismuth,  and 
the  metals  of  the  rare  earths.  Chemical 
processes,  which  it  would  take  too  long  to 
describe  here,  permit  the  extraction  of  the 
barium  with  the  radium,  the  bismuth  with 
the  polonium,  and  the  rare  earths  with  the 
actinium ;  after  this  there  remains  the  sepa- 
ration of  each  of  the  radio-active  bodies  from 
the  body  which  accompanied  it,  in  spite  of 
previous  chemical  reactions.  This  separa- 
tion, to  which  Mme.  Curie  has  devoted  many 
years  of  labour,  has  not  yet  been  completely 
successful,  except  in  the  case  of  radium, 
where  the  following  process  was  employed. 
The  radiferous  chloride  of  barium,  of 
which  about  eight  kilograms  is  extracted 
from  a  ton  of  pitchblende  residues,  is  dis- 
solved in  hot  water,  so  as  to  form  a  saturated 
solution.  On  cooling,  crystals,  which  we 
will  call  A,  are  thrown  down,  while  on  evap- 
orating the  remaining  solution,  another 


RADIO-ACTIVITY  67 

chloride,  which  we  will  call  B,  is  obtained. 
Since  the  chloride  of  radium  is  slightly  less 
soluble  than  the  barium  chloride,  it  results 
that  the  chloride  A  is  richer,  and  the  chloride 
B  is  poorer,  in  radium  than  the  chloride  used 
in  forming  the  solution.  This  may  be  ascer- 
tained by  determining  the  relative  radio-activ- 
ity. A  second  operation  exactly  similar  to 
the  above  is  carried  out  with  each  of  the 
products  A  and  B ;  now  since  the  least  active 
part  obtained  from  A  and  the  most  active 
part  obtained  from  B  are  about  equally 
radio-active,  they  are  reunited,  and  thus  the 
four  portions  which  result  from  the  second 
operation  reduce  to  three  of  different  rich- 
ness in  radium.  The  process  is  continued 
in  the  same  manner ;  however,  to  prevent  an 
inordinate  increase  in  the  number  of  por- 
tions, no  use  is  made  of  those  whose  radio- 
activity is  very  small,  and  the  operation  is 
discontinued  on  those  portions  whose  radio- 
activity is  sufficiently  great.  It  is  also  found 
advantageous  to  make  use  of  the  mother 
liquor  obtained  from  one  operation  to  dis- 
solve the  crystals  obtained  in  the  succeeding 


68  RADIO-ACTIVITY 

operation.  When  the  greater  part  of  the 
inactive  substances  is  eliminated  by  this  pro- 
cess, it  is  carried  on  still  further,  but  now  the 
least  active  parts  are  more  freely  discarded. 
In  this  way  a  small  amount  of  radium  is  lost, 
but  the  purification  proceeds  more  rapidly, 
to  accomplish  which  it  is  advisable  to  acidu- 
late the  solvent  more  and  more  with  pure 
hydrochloric  acid.  Finally,  practically  pure 
chloride  of  radium  is  separated  to  the  amount 
of  two  or  three  decigrams  per  ton  of  the 
residues  employed.  At  present  bromide  of 
radium  is  also  prepared  in  the  pure  state. 

Besides  the  measurement  of  the  degree  of 
increasing  radio-activity  a  spectroscopic  ex- 
amination of  the  product  may  be  employed 
to  estimate  its  purity,  since  radium  possesses 
a  spectrum  which  perfectly  characterizes  it 
and  which  was  studied  by  Dema^ay.  If  an 
impure  product  is  examined,  the  spectrum 
of  barium  appears  with  the  spectrum  of 
radium;  but  as  the  process  of  purification 
goes  on,  the  barium  lines  become  weaker 
and  finally  almost  completely  disappear. 

After  the  principal  effects  due  to  radio- 


RADIO-ACTIVITY  69 

active  bodies  were  discovered,  the  attention 
of  physicists  was  turned  to  the  direct  study 
of  the  rays  emitted. 

All  known  radiations  are,  or  at  least  we 
conceive  that  they  are,  of  two  sorts;  those 
due  to  waves  propagated  in  the  ether,  and 
those  due  to  the  motion  of  electrified  mate- 
rial particles.  Not  only  the  luminous  rays 
properly  so  called,  and  the  invisible  heat  and 
ultraviolet  rays,  but  also,  as  is  thought,  the 
Rontgen  rays  belong  to  the  first  sort.  To 
the  second  sort  belong  the  cathode  rays, 
which  are,  in  fact,  considered  to  be  due  to 
the  motion  of  negative  electrons. 

It  is  not  difficult  to  decide  on  the  nature 
of  new  radiations  which  belong  to  one  or  the 
other  of  these  two  categories ;  in  fact,  while 
an  electric  or  magnetic  field  cannot  in  the 
least  degree  modify  the  form  of  luminous  or 
of  Rontgen  rays,  etc.,  they  should  cause  a 
marked  curvature  of  the  path  described  by 
electrified  particles,  unless  the  velocity  is 
exceedingly  great.  However,  there  exists,  as 
it  appears,  certain  new  radiations  to  which 
their  discoverer,  Blondlot,  has  given  the  name 


70  RADIO-ACTIVITY 

of  n-rays  (35),  which  seem  to  possess  some 
of  the  properties  of  heat  rays,  but  which 
appear  to  possess  other  very  strange  proper- 
ties. We  shall  not  attempt  to  describe  them 
here,  since  their  nature  is  still  enigmatic. 

In  order  to  acquire  a  clear  conception  of 
the  nature  of  the  rays  emitted  by  radio-active 
bodies,  it  was  necessary  to  subject  them  to 
magnetic  or  electric  forces,  and  therefore  to 
place  the  radio-active  bodies  either  between 
the  poles  of  a  powerful  magnet  or  between 
two  metal  plates  oppositely  electrified.  More- 
over, in  order  that  the  possible  deformation 
of  the  rays  might  be  recognized,  they  had  to 
be  passed  through  a  small  opening  or  dia- 
phragm, on  the  far  side  of  which  either  a 
phosphorescent  body  or  a  photographic  plate 
protected  from  the  light  by  a  black  paper 
covering  was  placed.  With  the  phosphores- 
cent body  the  displacement  of  the  luminous 
spot  is  at  once  seen  when  an  electric  or  a 
magnetic  field  deforms  the  rays,  and  the 
same  result  is  detected  when  the  plate  is 
developed.  It  is  generally  preferable  to  use 
the  photographic  plate,  because  the  length 


RADIO-ACTIVITY  71 

of  exposure,  which  may  be  as  great  as  is 
desired,  eventually  compensates  for  the  low 
intensity  of  the  radiations  which  are  studied. 

Applying  these  methods,  it  was  recognized 
from  the  very  first  that  in  general  a  radio- 
active body  emits  rays  which  are  deviated 
by  the  magnetic  and  the  electric  field,  and 
at  the  same  time  other  rays  which  are  not 
deviated.  It  was  further  determined  that 
the  former  behave  in  all  respects  like  cathode 
rays  of  high  velocity,  that  is,  as  though  they 
consisted  of  negative  electrons  projected  in 
straight  lines  with  enormous  velocity.  We 
shall  see  further  on  that  this  velocity  may  be 
measured,  and  that  the  ratio  between  electric 
charge  and  mass  has  practically  the  same 
value  as  that  found  in  the  case  of  the  cathode 
rays. 

Subsequent  researches  have  proved  that 
radium  and  other  strongly  radio-active  bodies 
also  produce  rays  deviated  somewhat  less 
than  the  cathode  rays  by  the  electric  and 
magnetic  forces,  but  in  the  opposite  direc- 
tion. Hence  it  may  be  said  that  radio-active 
bodies  emit  three  varieties  of  rays.  It  is 


72  RADIO-ACTIVITY 

probable  that  this  is  true  of  all  bodies,  since, 
even  if  some  of  these  varieties  of  rays  have 
not  yet  been  found  in  the  complex  emission 
of  certain  radio-active  bodies,  this  is  in  all 
probability  due  to  their  being  less  intense 
and  hence  less  easy  to  detect.  An  example 
of  this  is  found  in  Mme.  Curie's  polonium, 
which  practically  only  emits  rays  whose 
deviation  is  opposite  to  that  of  the  cathode 
rays. 

The  rays  which  are  subject  to  deviation 
can  only  be  considered  as  consisting  in  the 
emission  of  electrified  particles.  The  direc- 
tion of  the  deviation  shows  the  sign  of  the 
charges  of  the  particles,  while  the  amount  of 
the  deviation  permits  an  evaluation  of  the 
velocities  with  which  the  particles  move,  and 
in  addition,  of  the  ratio  which  exists  between 
the  charge  and  the  mass  of  each;  conse- 
quently the  mass  itself  may  be  determined 
if  we  assume  that  its  electric  charge  has  that 
constant  value  which  pertains  to  the  electro- 
lytic ion  of  hydrogen.  We  will  now  sum  up 
the  results  of  the  research  relating  to  radio- 
activity, adopting  Rutherford's  designation, 


RADIO-ACTIVITY  73 

a,  /?,  y,  for  the  three  kinds  of  rays  emitted 
by  radium,  and  perhaps  in  general  by  every 
radio-active  body. 

The  a-rays  behave  in  a  manner  which 
confirms  the  hypothesis  made  by  Strutt 
(36),  which  is  that  they  consist  of  positive 
ions  projected  in  all  directions  from  the 
radio-active  body.  In  fact,  Rutherford  (37) 
proved  that  they  transport  positive  charges, 
while  Becquerel  (38)  determined  that  their 
deviation  in  a  magnetic  field  is  in  the  oppo- 
site direction  to  that  of  the  cathode  rays. 

If,  for  example,  the  rays  emanate  from  a 
small  quantity  of  one  of  the  salts  of  radium 
contained  in  a  little  lead  vessel,  P  (Fig.  12), 
they  are  propagated  in  the  direction  of  the 
straight  line  PC]  but  if  a  magnetic  field  is 
set  up  perpendicular  to  the  plane  of  the  fig- 
ure, the  a-rays  separate  themselves  from  the 
others  and  curve  in  the  direction  of  the  arc 
PA. 

It  was  found,  on  measuring  the  velocity 
and  the  ratio  between  the  charge  and  the 
mass  of  the  particles  constituting  the  a-rays, 
that  the  velocity  may  attain  a  value  about  a 


74 


RADIO-ACTIVITY 


tenth  of  the  velocity  of  light,  and  that  the 
ratio  mentioned  above  would  indicate  that 
the  particles  possess  a  mass  of  atomic  order. 
We  may  say,  therefore,  that  the  a-rays  are 
identical  with  canal  rays  of  high  velocity. 


FIG.  12. 


The  ionization  of  a  gas  seems  to  be  espe- 
cially due  to  the  a-rays,  evidently  as  a  result 
of  collisions.  The  phenomenon  is  rather 
restricted  if  the  gas  which  surrounds  the 
radio-active  body  is  at  ordinary  pressure. 
In  fact,  the  a-rays  from  radium  are  only 
slightly  penetrating,  as  either  a  layer  of 


RADIO-ACTIVITY  75 

ordinary  air  ten  centimeters  thick,  or  a  thin 
plate  of  aluminium  less  than  a  tenth  of  a 
millimeter  thick,  is  sufficient  to  absorb  the 
greater  part  of  the  rays. 

The  /3-rays  behave  in  all  respects  like  very 
penetrating  cathode  rays.  That  is,  they  are 
negative  electrons  projected  in  all  directions ; 
their  velocity  is  enormous,  since  it  may  attain 
a  value  but  slightly  inferior  to  the  velocity 
of  light.  This  result  is  deduced  from  the 
effect  produced  on  the  rays  by  the  magnetic 
field,  which  was  determined  almost  at  the 
same  time  by  Becquerel  (39),  Giesel  (40), 
Meyer  and  Von  Schweidler  (41),  and  Dorn 
(42).  Instead  of  curving  like  the  a-rays, 
they  bend  considerably  more  in  the  opposite 
direction,  as  is  shown  in  Figure  12.  The  cur- 
vature of  some  of  the  /2-rays  may  be  so  great 
that  they  end  by  striking  a  photographic 
plate,  ZZ,  on  which  the  lead  vessel  P  rests 
in  the  region  B^  B%. 

The  production  of  rays  curved  to  a  greater 
or  less  degree  by  the  magnetic  field  is  due 
to  the  fact  that  the  velocities  of  the  various 
negative  electrons,  of  which  the  /3-rays  are 


76  RADIO-ACTIVITY 

only  the  trajectory,  have  different  values. 
The  magnetic  field  causes  those  which  move 
less  rapidly  to  describe  semicircles  of  small 
radius  and  those  with  the  high  velocity  to 
describe  the  arcs  of  large  radius.  It  is  thus 
that  the  elongated  image  obtained  on  the 
photographic  plate  is  explained.  By  means 
of  this  image  it  is  easy  to  prove  that  the  less 
deflected  rays,  consisting  of  electrons  with 
great  velocity,  are  also  the  more  penetrating. 
In  fact,  a  photographic  plate  placed  in  the 
path  of  the  /8-rays  absorbs  most  strongly 
those  rays  which  are  the  most  deviated  (B^ 
Fig.  12).  Moreover,  the  variety  of  /3-rays  is 
quite  great;  thus,  while  some  are  stopped 
by  aluminium  foil  one-hundredth  of  a  milli- 
meter thick,  others  are  able  to  traverse  sev- 
eral millimeters  of  lead.  It  was  proved 
directly  by  the  Curies  (43)  that  the  /8-rays 
really  transport  negative  charges.  In  their 
experiments  the  a-rays  were  arrested  by  a 
plate  of  aluminium  so  that  only  the  effect  of 
the  /2-rays  was  shown  by  the  electrometer. 

While  the  a  and  (3  rays  are  deviated  in  an 
electric  or  a  magnetic  field,  the  y-rays   are 


RADIO-ACTIVITY  77 

not  affected  at  all.  Thus  those  emitted  by 
the  radio-active  body  contained  in  P  (Fig. 
12)  preserve  their  rectilinear  motion,  PC, 
even  when  the  magnetic  field  is  set  up. 

The  y-rays,  like  the  y6-rays,  are  not  homo- 
geneous, there  being  a  more  penetrating  and 
a  less  penetrating  variety.  This  property 
renders  them  very  similar  to  the  Rontgen 
rays,  and  as  such  they  are  now  generally 
considered.  It  is  true  it  has  been  observed 
that  the  conductivity  excited  by  the  y-rays 
in  various  gases  is  not  proportional  to  that 
produced  by  the  X-rays,  and  this  seemed  to 
establish  a  difference  in  the  nature  of  the 
two  sorts  of  rays.  But  recent  experiment 
has  shown  that  this  dissimilarity  in  behav- 
iour depends  solely  on  the  fact  that  the  y-rays 
in  the  aggregate  are  only  comparable  with 
the  most  penetrating  X-rays.  In  fact,  the 
ratio  between  the  conductivity  produced  in 
various  gases  by  y  and  X  rays  tends  toward 
unity,  when  a  comparison  is  made  between 
the  former  and  Rontgen  rays  which  are 
furnished  by  "  hard  "  tubes  and  are  made  to 
pass  through  a  lead  plate  before  arriving  at 
the  gas  to  be  ionized. 


78  RADIO-ACTIVITY 

When  the  three  sorts  of  rays  strike  or 
traverse  various  bodies,  they  produce  effects 
which  differ  according  to  the  nature  of  the 
bodies  themselves.  This  is  particularly  evi- 
dent when  one  of  the  salts  of  radium  is  em- 
ployed and  certain  of  these  effects  have  only 
thus  far  been  observed  with  this  substance. 
It  is  not  possible  to  completely  isolate  the 
rays  of  one  species  from  the  others  and  to 
study  separately  the  phenomena  produced  by 
them ;  but  it  is  possible,  by  means  of  the  in- 
terposition of  absorbing  plates  to  keep  back 
the  least  penetrating  rays,  such  as  the  a,  or 
the  a  together  with  a  part  of  the  ft  rays,  or 
finally,  to  allow  only  the  y-rays  to  pass 
through ;  and  this  is  sufficient  in  many  cases 
to  allow  the  effects  produced  by  the  one  or 
the  other  to  be  recognized. 

The  effects  produced  by  radio-active  bodies, 
and  in  particular  by  radium,  may  be  classified 
as  luminous,  chemical,  electrical,  mechanical, 
thermal,  and  physiological.  Besides  what  has 
been  previously  stated,  the  following  may  be 
said  with  reference  to  these  effects. 

Phosphorescence  and  fluorescence  seem  to 


RADIO-ACTIVITY  79 

result  especially  from  the  action  of  the  a  and 
the  ft  rays ;  but  certain  bodies  become  more 
brilliantly  luminescent  when  they  are  struck 
by  the  a-rays,  others  when  they  are  struck  by 
the  /3-rays.  For  example,  hexagonal  blende 
is  particularly  luminescent  under  the  action 
of  the  a-rays  from  radium. 

Crookes  has  constructed  a  small  instru- 
ment called  the  spinthariscope  (44),  by  means 
of  which  the  effect  produced  by  radium  on  a 
screen  coated  with  phosphorescent  sulphide 
of  zinc  is  observed.  A  small  particle  of  the 
radium  salt  is  placed  at  a  distance  of  about 
a  half  millimeter  from  the  screen,  which  is 
viewed  through  a  lens  or  microscope.  Brill- 
iant points  appear  here  and  there  and  imme- 
diately become  extinguished  and  thus  give 
an  effect  of  scintillation.  According  to 
Crookes  each  luminous  point  is  caused  by 
the  impact  of  a  positive  ion.  According  to 
Becquerel,  the  scintillation  is  due  to  the  rup- 
ture or  cleavage  of  the  little  crystals  forming 
the  phosphorescent  films,  since  he  obtained 
similar  effects  by  crushing  the  body  between 
two  glass  plates  (45).  In  any  case  the  singu- 


80  RADIO-ACTIVITY 

lar  luminous  phenomenon  of  the  spinthari- 
scope is  only  indirectly  due  to  the  supposed 
bombardment  of  the  phosphorescent  body  by 
the  a-rays. 

The  luminosity  of  the  salts  of  radium  has 
been  attributed  to  a  phosphorescence  excited 
by  the  rays  which  they  themselves  generate. 
But  it  may  be  that  the  light  is  produced  by 
the  collision  of  the  a  ions  or  of  the  /8  elec- 
trons, not,  in  fact,  with  the  molecules  of  the 
radio-active  body,  but  with  those  of  the  sur- 
rounding gas  or  of  the  gases  which  are  slowly 
released  from  the  radium  salt;  and  it  might 
also  be  supposed  that  the  cause  of  the  lumi- 
nous phenomenon  is  the  transformation  of 
the  radium  into  an  emanation,  of  which  we 
shall  speak  later.  However,  it  has  been 
found  by  Sir  William  and  Lady  Huggins  (46) 
that  the  spectrum  of  the  light  emitted  by 
radium  consists  of  lines  coincident  with 
some  of  those  of  nitrogen,  and  almost  iden- 
tical with  that  of  the  negative  glow  produced 
in  air  at  ordinary  pressure. 

Recently  Becquerel  (47)  has  observed  that 
the  salts  of  uranium  are  also  spontaneously 


RADIO-ACTIVITY  8 1 

luminous.  In  order  to  convince  oneself  of 
this,  it  is  only  necessary  to  look,  at  night, 
when  the  eye  is  well  rested,  at  a  small  glass 
vessel  containing  uranium  nitrate.  The 
writer  has,  under  such  conditions,  observed 
a  very  vivid  scintillation  when  the  vessel  con- 
taining the  salt  was  shaken ;  for  this  reason 
it  is  advisable  not  to  shake  the  vessel  too 
strongly  if  one  wishes  to  verify  the  sponta- 
neous emission  of  light  and  not  confound  it 
with  that  which  is  produced  by  concussions, 
and  which  is  probably  electrical  in  origin. 

Phosphorescent  substances  are  altered  by 
the  action  of  the  rays  emitted  by  radium  and 
often  change  colour ;  glass  becomes  violet  or 
blackish,  and  thus  modified,  it  is  thermo- 
luminescent,  since  it  becomes  luminous  when 
heated  to  about  500°  C.  These  modifications 
are  probably  chemical  in  nature,  like  those 
to  which  images  on  photographic  plates  are 
due. 

Inasmuch  as  the  rays  from  radium  are 
unequally  absorbed  by  bodies  of  different 
density,  it  is  possible  by  their  use  to  obtain 
radiographs  similar  to  those,  now  so  well 


82  RADIO-ACTIVITY 

known,  which  are  obtained  with  X-rays. 
However,  the  latter  are  always  very  much 
the  more  perfect. 

Radio-active  bodies,  and  especially  radium, 
produce  still  other  chemical  actions ;  thus 
according  to  Becquerel,  a  solution  of  bichlo- 
ride of  mercury  and  oxalic  acid  (6J  grams 
of  the  bichloride,  12^  of  oxalic  acid,  and 
100  of  water)  deposits  calomel  under  the 
action  of  the  rays  from  radium ;  their  action 
is  therefore  in  this  case  similar  to  that  of 
light  rays.  Under  the  action  of  the  rays 
from  radium  ordinary  phosphorus  is  trans- 
formed into  red  phosphorus,  and  oxygen  into 
ozone.  And  it  seems  as  though  the  changes 
in  colour  which  are  shown  by  glass,  barium- 
platino-cyanide,  etc.,  should  be  attributed  to 
chemical  modifications.  The  last  of  these 
substances  becomes  at  the  same  time  less 
phosphorescent,  but  later  reacquires  this 
property  when  exposed  to  sunlight.  It  is 
a  rather  singular  fact  that  the  rays  from 
radium  do  not  sensibly  modify  the  iodide 
of  silver  of  a  daguerreotype  plate. 

Since  the  a  and   /3  rays    consist   in   the 


RADIO-ACTIVITY  83 

emission  of  positive  ions  and  negative  elec- 
trons respectively,  it  is  natural  that  they 
should  produce  an  electric  charge  whenever 
the  one  variety  and  not  the  other  is  arrested. 
In  this  manner  radio-active  substances  be- 
come a  continuous  source  of  electricity. 
Radio-active  bodies  should  become  electri- 
fied even  when  free  motion  is  allowed  to  all 
the  rays,  if  the  quantity  of  positive  electricity 
transported  by  the  a-rays  differs  from  the 
quantity  of  negative  electricity  carried  away 
in  the  same  time  by  the  /3-rays.  Appropriate 
experiments  have  shown  that  this  does  not 
happen  (48);  and  hence  there  must  be  a 
compensation  between  the  charges  of  the  two 
signs.  If,  however,  the  a-rays  are  arrested 
by  a  sheathing  of  not  too  thin  glass,  this 
becomes  positively  electrified.  This  experi- 
ment was  made  by  Wien  with  a  small 
amount  of  bromide  of  radium  placed  in  a 
glass  tube  wrapped  in  aluminium  foil  and 
hung  by  a  wire  in  a  vacuum  tube.  The 
gas  must  be  removed  from  the  region  sur- 
rounding the  small  tube  in  order  that  the 
conductivity  of  the  ionized  gas  may  not  cause 


RADIO-ACTIVITY 


the  charge  which  collects  to  be  dissipated. 
Under  some  conditions  an  electric  spark 
may  be  obtained,  as,  for 
example,  when  a  small  tube 
containing  radium  (49)  is 
opened. 

An  ingenious  little  piece 
of  apparatus  (Fig.  13)  which 
was  devised  by  Strutt  (50) 
demonstrates  the  continual 
production  of  electricity  by 
radium.  In  a  hermetically 
sealed  glass  vessel,  in  which 
a  good  vacuum  has  been 
formed,  a  small  closed  tube, 
a,  is  placed  which  contains 
a  radio-active  body.  This 
tube,  which  is  supported  by 
an  insulator  of  fused  quartz, 
6,  has  its  surface  covered 
with  a  conducting  film  (for 
example,  painted  with  phos- 
phoric acid),  and  terminates 
in  two  gold  leaves,  cc,  which  form  an  electro- 
scope. These  leaves  are  always  in  motion. 


FIG.  13. 


RADIO-ACTIVITY  85 

They  spread  apart  little  by  little  until  they 
touch  the  strips  of  tin  foil,  ee,  pasted  on  the 
walls  of  the  vessel  and  communicating  with 
the  earth.  Then  they  instantly  fall  together, 
only  to  slowly  diverge  again,  and  this  goes 
on  continuously.  The  effect  is  due  to  the 
positive  charge  left  on  the  little  tube  by  the 
a-rays  which  are  absorbed  while  the  negative 
charge  is  carried  away  by  the  /3-rays. 

Another  effect  of  an  electrical  nature, 
which  is  due  to  the  rays  from  radium,  is 
obtained,  according  to  Curie  (51),  in  the 
following  manner:  conductors  are  placed 
in  such  a  way  that  the  discharge  of  an 
electric  machine  or  an  induction  coil  may 
follow  two  separate  paths,  in  each  of  which 
an  air  gap  is  provided  for  the  formation  of 
the  sparks.  If  these  two  air  gaps  are  equiv- 
alent, so  that  the  discharge  takes  place  equally 
well  in  either  of  the  two  paths  open  to  it,  it 
is  sufficient  to  bring  the  radium  near  one  of 
the  two  air  gaps  in  order  that  the  spark  may 
form  at  this  one  and  not  at  the  other.  There- 
fore the  rays  from  radium  produce  an  action 
similar  to  that  which,  in  certain  circum- 


86  RADIO-ACTIVITY 

stances,  is  caused  by  the  Rontgen  rays,  in 
that  they  tend  to  assist  the  formation  of 
sparks. 

Finally,  another  electrical  effect  of  radium 
is  the  one  which  has  been  recently  an- 
nounced, namely,  that  a  diminution  in  the 
electrical  resistance  of  bismuth  is  produced 
when  the  radio-active  substance  is  brought 
to  within  a  very  small  distance  of  a  thin 
plate  of  this  metal. 

Since  the  a-rays  consist  of  positive  ions,  it 
follows  that  a  body  which  emits  them  must 
necessarily  send  out  matter  as  well ;  from  a 
certain  standpoint  the  same  thing  may  be 
said  of  the  ^8-rays,  as  we  shall  see  later.  It  is 
to  be  expected,  therefore,  that  there  should 
be  a  continual  diminution  in  the  weight  of 
radio-active  substances.  Heydweiller  (52) 
thought  that  he  had  detected  this  ;  but  Dorn 
(53)  could  not  confirm  this  result  in  spite  of 
the  fact  that  he  employed  a  body  of  greater 
radio-activity.  Thus  it  would  seem  that  we 
are  dealing  with  a  decrease  in  mass  so  small 
as  to  elude  ordinary  determinations. 

In  the  case  of  radium,  the  emission  of  the 


RADIO-ACTIVITY  87 

various  sorts  of  rays  is  accompanied  by  a 
continual  production  of  heat.  In  fact,  the 
Curies  and  Laborde  (54)  found  that  a  small 
quantity  of  a  radium  salt  continually  main- 
tained itself  at  a  higher  temperature  than 
that  of  the  surrounding  air.  The  experiment, 
which  is  quite  simple,  is  thus  described  by 
Mme.  Curie.  A  closed  glass  flask  contain- 
ing seven  decigrams  of  pure  bromide  of 
radium  and  a  mercury  thermometer  are 
placed  inside  a  double-walled  glass  vessel 
closed  with  a  cotton  stopper.  A  vacuum  is 
produced  between  the  two  walls  of  the  vessel, 
which  is,  in  fact,  one  of  those  which  are  used 
to  preserve  liquid  air  because  of  the  good 
heat  insulation  which  may  be  obtained  in  this 
manner.  A  precisely  similar  thermometer, 
together  with  a  similar  glass  flask,  which, 
however,  contains  an  inactive  body  such  as 
chloride  of  barium,  instead  of  the  radio-active 
substance,  are  placed  in  a  second  glass  vessel 
similar  to  the  first.  As  soon  as  thermal  equi- 
librium is  established,  it  may  be  observed  that 
the  first  thermometer  registers  a  temperature 
about  three  degrees  higher  than  the  second. 


88  RADIO-ACTIVITY 

In  order  to  measure  the  quantity  of  heat 
continuously  evolved  by  the  radium  salt,  a 
small  quantity  of  it  was  introduced  into  a 
Bunsen  calorimeter.  From  the  quantity  of 
ice  which  melted  in  a  given  time,  it  was 
deduced  that  the  heat  evolved  by  the  radio- 
active body  in  one  hour  would  be  sufficient 
to  melt  a  weight  of  ice  equal  to  its  own,  or 
to  heat  an  equal  weight  of  water  through 
about  80°  C.  Thus  we  see  that  the  quan- 
tity of  heat  furnished  by  radium  is  relatively 
great. 

A  thermal  effect  of  another  sort  is  that 
demonstrated  by  Georgiewski  (55).  Accord- 
ing to  this  author,  while  the  velocity  with 
which  a  hot  body  cools  does  not  vary  when 
the  gas  which  surrounds  it  is  ionized  by  the 
rays  from  radium,  the  cooling  itself  is  accel- 
erated if  the  hot  body  is  electrified,  and  espe- 
cially if  it  is  negatively  electrified.  This 
effect  was  observed  not  only  with  air,  but 
also  with  other  gases,  and  the  result  was  the 
same  whether  all  the  rays  from  radium  were 
allowed  to  act,  or  whether  the  a-rays  were 
prevented  from  acting. 


RADIO-ACTIVITY  89 

When  the  rays  from  radium,  and  from 
radio-active  bodies  in  general,  strike  an  inac- 
tive body,  they  cause  it  to  emit  new  rays 
which  are  called  secondary  rays.  This  emis- 
sion is  contemporaneous  with  the  action  of  • 
the  incident  rays,  which  distinguishes  this 
phenomenon  from  that  of  induced  radio- 
activity, of  which  we  shall  speak  farther  on. 
The  property  of  exciting  secondary  rays  per- 
tains also  to  other  radiations,  such  as  X-rays 
and  cathode  rays,  and  in  all  cases  the  second- 
ary rays  differ  from  those  which  cause  them. 
Thus  it  may  be  said  that  the  X-rays  are  sec- 
ondary rays  produced  by  cathode  rays,  which 
may  themselves  be  generated  by  X-rays  (56). 
Moreover,  these  latter  produce  secondary 
rays  of  a  nature  similar  to  their  own  but 
which  are  less  penetrating.  Luminous  and 
ultraviolet  rays  may  produce  cathode  rays 
as  secondary  rays,  etc.  The  rays  from  radio- 
active bodies  produce  effects  of  this  sort, 
which,  however,  have  not  yet  been  studied 
in  a  thorough  manner. 

Besides  the  effects  already  described,  the 
rays  emitted  by  radium  have  a   most  pro- 


90  RADIO-ACTIVITY 

nounced  action  on  living  tissues  and  may 
even  cause  severe  lesions  hard  to  be  cured. 
Becquerel  and  the  Curies  proved  this  to 
their  cost  (57).  According  to  Danysz  (58) 
a  tube  containing  a  strongly  radio-active 
body  placed  on  the  skin  of  an  animal  caused 
a  complete  destruction  of  the  epidermis  and 
the  dermis,  though  the  action  on  the  lower 
tissues  was  relatively  quite  weak.  The  ner- 
vous system  is,  on  the  contrary,  quite  sensi- 
tive to  the  action  of  radium,  since  lesions  are 
soon  formed  which  may  result  in  paralysis 
and  death. 

The  effect  produced  on  bacteria  is  worthy 
of  note;  for,  according  to  Aschkinass  and 
Caspari  (59),  the  development  of  certain 
species  becomes  arrested  when  they  are  sub- 
jected to  these  rays.  The  larvae  of  some 
insects  die,  and  the  prolonged  action  of  the 
rays  from  radium  on  the  seeds  of  cer- 
tain plants  deprives  them  of  the  ability  to 
germinate.  It  seems,  moreover,  that,  in 
compensation  for  the  deleterious  actions 
which  they  exert,  the  rays  emitted  by  radio- 
active substances  may  become  useful  in  the 


RADIO-ACTIVITY  91 

cure  of  certain  diseases,  such  as  cancer  and 
lupus. 

The  sensation  of  light  which  suffuses  the 
field  of  vision  when  one  of  the  radium  prepa- 
rations is  brought  close  to  the  eye  is  not  a 
direct  physiological  effect,  but  one  which  is 
explained  by  the  phosphorescence  of  the 
parts  which  constitute  the  organ  of  sight. 
The  sensation  is  produced  even  in  the  eye 
of  a  blind  person,  provided  the  retina  and 
the  optic  nerve  are  in  normal  condition. 

Finally,  according  to  Bloch  (61),  the  action 
of  the  rays  from  radium  on  crystalline  sele- 
nium is  similar  to  that  of  light,  since  they 
increase  its  electrical  conductivity. 

In  addition  to  the  emission  of  a,  ft,  and  y 
rays  radio-active  bodies,  or  at  least  radium 
and  thorium,  continually  lose  a  part  of  their 
substance  in  another  form.  Certain  experi- 
ments relative  to  the  ionization  of  the  air 
produced  by  thorium  compounds  led  Ruther- 
ford to  the  discovery  of  the  radio-active  ema- 
nations. Before  this  Owens  (62)  had  noticed 
a  strange  irregularity  in  the  ionizing  action 
of  the  rays  from  thorium  oxide,  but  it  was 


92  RADIO-ACTIVITY 

Rutherford  who  furnished  the  explanation 
of  this  (63)  by  demonstrating  that  thorium 
continually  emits  radio-active  particles,  whose 
ionizing  action  reenforces  that  due  to  the 
a,  y8,  and  y  rays.  Radium  itself  produces  its 
own  emanation,  and  the  particles  which  form 
this  emanation  are  distinct  from  those  which 
constitute  the  a  and  ft  rays,  since  the  former 
diffuse  slowly  like  the  molecules  of  a  gas 
while  the  latter  are  projected  with  enormous 
velocity. 

In  fact,  the  emanation  mixes  with  the  sur- 
rounding gas  and  may  be  carried  with  it  from 
one  place  to  another.  Recent  experiments 
show  that  the  emanation  behaves  like  a  true 
gas,  since,  according  to  Traubenberg  (64),  not 
only  the  emanation  contained  in  the  city 
water  of  Freiburg  (and,  in  general,  in  water 
from  springs  containing  any  radio-active  sub- 
stances whatsoever),  but  also  the  emanation 
from  radium  obeys  the  laws  of  Dalton  and 
Henry,  which  hold  for  gases. 

The  emanation  easily  passes  through  nar- 
row apertures  or  fissures  through  which  an 
ordinary  gas  could  only  circulate  with  ex- 


RADIO-ACTIVITY  93 

treme  slowness.  It  is  only  temporarily  radio- 
active ;  that  is,  its  radio-activity  decays  in  a 
continuous  manner  to  such  an  extent,  that  at 
the  end  of  a  minute  the  activity  of  the  tho- 
rium emanation  is  reduced  to  one-half  of  the 
initial  value,  while  that  of  radium  loses  its 
activity  considerably  less  rapidly,  being  re- 
duced to  one-half  only  after  about  four  days. 

Rutherford  and  Soddy  (65)  have  shown 
that  the  radium  emanation  condenses  at  a 
temperature  of  about  1 50°  below  zero  centi- 
grade. In  fact,  a  current  of  gas  which  has 
passed  over  a  compound  of  radium  and  into 
a  tube  surrounded  by  liquid  air,  issues  from 
the  tube  entirely  robbed  of  the  emanation, 
and  the  emanation  itself  returns  to  the  gas- 
eous state  when  the  temperature  is  raised  to 
the  value  mentioned  above.  It  is  easy  to 
follow  the  motion  of  the  emanation,  because 
it  is  luminous  and  renders  the  glass  tube 
which  contains  it  luminous.  The  emanation 
from  thorium  condenses  at  a  slightly  higher 
temperature. 

The  emanation  has  the  property  of  render- 
ing bodies  with  which  it  comes  in  contact 


94  RADIO-ACTIVITY 

temporarily  active.  The  radio-activity  ac- 
quired in  this  manner  by  bodies  which  do 
not  possess  it  on  their  own  account  was,  it 
seems,  first  observed  by  the  Curies,  who 
called  it  induced  radio-activity.  According 
to  Rutherford  it  is  due  to  an  invisible  solid 
substance  which  the  emanation  deposits  on 
bodies  in  unappreciable  quantities,  and  which 
is  soluble  in  certain  acids  but  not  in  others. 
Upon  evaporating  the  solution  formed  in 
this  way,  a  radio-active  residue  is  obtained. 
Moreover,  it  seems  that  the  emanation  ad- 
heres to  the  bodies  which  it  has  rendered 
active,  and  also,  in  a  certain  sense,  is  ab- 
sorbed by  them,  since  these  bodies  give  off 
emanations  of  their  own.  Celluloid,  india 
rubber,  and  paraffin  absorb  and  then  give  up 
the  emanation  in  appreciable  quantities.  In- 
dependently of  this  it  has  been  found  that 
a  body  which  has  become  radio-active  by 
induction,  in  a  large  measure  loses  this  prop- 
erty when  it  is  heated;  this  lost  activity  is 
acquired  by  surrounding  cold  bodies. 

The  presence  of  a  negative  charge  on  a 
body  facilitates   the  production   of   induced 


RADIO-ACTIVITY  95 

radio-activity.  It  is  even  possible  to  cause 
a  conductor  which  is  strongly  charged  nega- 
tively to  become  radio-active  by  merely  plac- 
ing it  for  several  hours  in  the  open  air,  as 
has  been  shown  by  Elster  and  Geitel  (67). 
According  to  Sella  (68)  the  same  result  may 
be  obtained  in  certain  cases  when  the  con- 
ductor is  charged  positively.  These  two 
experiments  point  to  the  existence  in  the 
atmosphere  of  an  emanation  analogous  to 
those  which  are  given  off  by  radium  and 
thorium. 

Radio-activity  may  also  be  produced  in 
substances  which  in  themselves  are  inactive, 
by  dissolving  the  inactive  body  in  a  liquid 
together  with  an  active  body  and  then  sepa- 
rating the  two  by  chemical  methods.  In 
this  manner  active  bismuth  is  obtained  after 
having  been  dissolved  in  a  solution  contain- 
ing radium.  Generally  the  body  which  has 
become  radio-active  in  this  way  slowly  loses 
its  activity ;  and  it  is  because  of  this  that 
polonium  has  been  considered  by  some  to  be 
bismuth  which  has  been  rendered  active  by 
solution.  A  salt  of  barium  becomes  radio- 


96  RADIO-ACTIVITY 

active,  in  a  similar  way,  when  it  is  derived 
from  a  solution  of  uranium,  while  this  solu- 
tion loses  a  part  of  its  radio-activity,  which 
it  later  slowly  regains. 

A  phenomenon  analogous  to  this,  and 
one  to  which  we  shall  return  later,  is  that 
described  by  Rutherford,  who  was  able  to 
chemically  separate  thorium  into  two  por- 
tions ;  the  one  which  he  called  thorium-X  is 
quite  active  and  loses  its  radio-activity  little 
by  little  as  time  goes  on,  while  the  other  in 
time  reacquires  this  lost  property. 

The  continued  evolution  of  heat  and  the 
emission  of  a-rays  as  well  do  not  pertain 
exclusively  to  radium,  but  are  also  properties 
of  the  emanation  which  it  evolves.  If  a 
radium  salt  is  heated  in  a  tube  until  so  much 
of  its  emanation  as  it  is  possible  to  liberate 
is  driven  off  and  collected  separately  in  a 
tube  cooled  by  liquid  air,  then,  when  the  tube 
containing  the  radium  and  that  containing 
the  emanation  come  into  thermal  equilibrium 
with  the  surrounding  atmosphere,  it  is  found 
that  the  quantity  of  heat  produced  in  unit 
time  by  the  radium  diminishes,  while  that 


RADIO-ACTIVITY  97 

evolved  by  the  emanation  increases,  in  such 
a  way  that  the  sum  of  the  two  remains  prac- 
tically constant  (69).  If  the  emission  of  a- 
rays  on  the  part  of  the  radium  and  on  that  of 
the  emanation  are  compared  by  means  of  the 
ionization  which  they  produce,  a  behaviour 
exactly  similar  to  that  in  the  case  of  the 
emission  of  heat  is  noted.  This  leads  one  to 
suppose  (70)  that  the  heat  evolved  by  the 
radium  is  due  to  the  collision  of  the  positive 
ions  (a-rays)  with  the  air  molecules  or  with 
those  of  the  radio-active  substance  itself. 
The  velocity  of  the  a-rays  which  has  been 
found  to  attain  a  value  of  one-tenth  of  the 
velocity  of  light,  is  such  as  to  satisfy  this 
hypothesis ;  but  at  any  rate  the  origin  of  the 
energy,  of  which  radium  seems  to  be  an  inex- 
haustible source,  still  remains  unexplained. 

It  is  probable  that  radio-activity  is  a  prop- 
erty possessed  to  a  greater  or  less  degree  by 
all  bodies.  In  fact,  recent  experiments  have 
shown  that  the  speed  with  which  a  charged 
electroscope,  for  example  that  shown  in  Fig- 
ure 10,  becomes  discharged,  depends  on  the 
nature  of  the  walls  of  the  vessel  in  which  it 


98  RADIO-ACTIVITY 

is  inclosed  (71).  This  is  attributed  to  the 
radio-activity  of  the  walls  themselves.  It 
seems,  moreover,  that  very  penetrating  radia- 
tions, which  proceed  perhaps  from  the  atmos- 
phere or  from  surrounding  bodies,  continually 
exist  all  about  us  and  produce  a  slight  ioniza- 
tion  of  gases,  since  the  rapidity  of  discharge 
mentioned  above  becomes  less,  when  the  ap- 
paratus is  surrounded  by  a  thick  shield  of 
lead  (72).  Thus  one  might  even  suspect 
that  the  slight  radio-activity  of  ordinary  sub- 
stances may  be,  at  least  in  part,  an  emission 
of  secondary  rays  excited  by  these  radia- 
tions, which  are  supposed  to  be  continually 
present. 

The  fact  that  air  which  has  been  drawn  up 
from  beneath  the  soil  has  an  electrical  con- 
ductivity greater  than  that  of  ordinary  air  is 
especially  important.  Moreover,  it  has  been 
found  that  the  water  from  many  springs  and 
wells,  in  particular  the  water  supplied  to  the 
city  of  Cambridge  (73),  contains  a  radio-active 
gas  which  impregnates  air  made  to  bubble 
through  the  water  and  causes  it  to  become 
feebly  conducting.  These  two  facts  lead  one 


RADIO-ACTIVITY  99 

to  suppose  with  Elster  and  Geitel  (74)  that  a 
small  quantity  of  some  radio-active  substance, 
which  may  possibly  be  radium,  exists  in  the 
ground,  and  that  the  emanation  from  this  sub- 
stance is  carried  by  the  ground  air  and  is 
dissolved  to  a  slight  extent  by  spring  water. 
Mr.  Allen  (75),  who  discovered  that  the 
water  of  the  spring  called  "  King's  Bath  "  is 
radio-active,  advances  the  idea,  worthy  of  be- 
ing taken  into  consideration  in  this  connec- 
tion, that  the  therapeutic  action  of  mineral 
waters  may  be  due  at  least  in  part  to  an  em- 
anation or  to  a  radio-active  gas  contained  in 
them ;  thus  it  would  be  easy  to  account  for 
the  fact,  which  is  so  often  asserted,  even  if  it 
has  never  been  explicitly  demonstrated,  that 
mineral  waters  become  less  efficacious  when 
exported. 

The  presence  of  radio-active  emanations 
in  springs  which  contain  carbonic  acid  gas, 
and  in  certain  thermal  waters,  as  well  as  the 
relatively  great  radio-activity  of  the  mud 
deposited  from  these  waters,  leads  one  to 
suppose  that  the  proportion  of  radium,  or  of 
some  other  very  radio-active  substance,  con- 


100  RADIO-ACTIVITY 

tained  in  the  earth's  crust,  must  increase  with 
the  depth  below  the  surface  (76).  Hence  it 
would  seem  very  well  worth  while  to  insti- 
tute an  investigation  of  volcanic  products. 

Freshly  fallen  snow  is  radio-active.  The 
writer  has  been  able  to  ascertain  that  the 
conductivity  of  the  air  on  a  snowy  day  was 
more  than  double  that  on  ordinary  days. 

In  this  chapter  we  have  presented  a  long 
series  of  facts  which  have  been,  for  the  most 
part,  ascertained  with  absolute  certainty,  but 
which  at  any  time  may  assume  a  different 
aspect  in  the  light  of  some  future  discovery. 
It  would,  therefore,  be  premature  to  formu- 
late a  theory  of  radio-activity  at  the  present 
time,  were  it  not  that  the  nature  of  this 
phenomenon  has  been  rendered  so  manifest 
by  the  properties  which  the  radiations  and 
emanations  have  been  found  to  possess.  In- 
deed, it  is  impossible  to  doubt  that  a  radio- 
active body  continually  emits  part  of  its  own 
substance  and  that  hence  it  should  have  a 
limited  existence :  the  electron  theory,  in  the 
form  in  which  it  will  be  presented  in  the  last 
chapter,  and  according  to  which  atoms  are 


RADIO-ACTIVITY  IOI 

systems  of  electrons,  seems  designed  ex- 
pressly to  confirm  this  belief.  But  a  theory 
of  radio-activity  should  also  explain  the 
origin  of  the  energy  which  becomes  avail- 
able during  the  disintegration  of  radio-active 
bodies. 

It  is  thought  by  some  that  this  energy 
comes  from  an  unknown  radiation  which  is 
passing  continually  through  all  space.  This 
radiation  might  impart  enormous  velocities 
to  the  constituents  of  the  atoms  of  certain 
bodies  —  velocities  great  enough  to  separate 
these  constituents  from  each  other.  Prob- 
ably if  such  a  radiation  really  did  exist  it 
would  be  absorbed  by  radio-active  bodies. 
Thus  it  would  seem  that  the  radio-activity  of 
a  body  might  be  reduced  by  surrounding  it 
with  other  radio-active  bodies.  Probably  no 
one  has  ever  thought  of  experimenting  in 
just  this  way,  but  Elster  and  Geitel  (77)  have 
found  that  the  radio-activity  of  a  body  does 
not  diminish  when  it  is  placed  underground, 
for  example  in  a  mine,  so  that  it  is  covered 
by  a  layer  of  the  earth's  crust  at  least  eight 
hundred  meters  thick ;  and  it  is  hardly  prob- 


102  RADIO-ACTIVITY 

able  that  this  should  not  absorb,  at  least  to 
an  appreciable  degree,  the  radiations  which 
are  supposed  to  pass  through  space. 

Hence  it  seems  preferable  to  adopt  the 
hypothesis  that  the  origin  of  the  energy  of 
radio-active  bodies  is  analogous  to  that  of  the 
thermal  energy  furnished  by  chemical  com- 
binations, with  the  difference  that,  while  in 
the  latter  case  we  are  dealing  with  the  atoms 
which  taken  in  the  free  state  or  issuing  from 
other  combinations  unite  together  to  form 
new  molecules,  in  the  case  of  the  radio-active 
bodies  we  are  dealing  instead  with  electrons 
derived  from  unstable  atoms,  and  which  unite 
to  form  new  and  more  enduring  atoms. 
Certain  experiments,  and  especially  those  of 
Rutherford  and  of  his  collaborators,  render 
these  atomic  modifications  and  transforma- 
tions highly  probable  and  lead  to  the  fol- 
lowing consideration  of  the  phenomena  of 
radio-activity  (78). 

The  atoms  of  the  radio-active  bodies  are 
unstable  systems  of  electrons.  Every  now 
and  then  some  of  these  atoms  subdivide  into 
free  negative  electrons  and  groups  of  elec- 


RADIO-ACTIVITY  103 

trons  with  positive  properties,  or  positive 
ions.  The  former  constitute  the  ^8-rays ;  the 
latter  the  a-rays.  The  emanation  probably 
consists  of  these  same  positive  ions  or  of 
modifications  of  them.  If  one  portion  only 
of  the  disintegrated  atoms  is  radiated  out 
into  the  surrounding  space,  the  remaining 
portion  will  constitute  a  new  body,  which 
may  be  itself  radio-active;  and  in  this  case 
its  atoms  will  be  subject  to  further  division. 
The  same  thing  may  be  said  of  the  new 
atoms  which  constitute  the  emanation,  as 
well  as  of  the  substance  which  this  emana- 
tion deposits  on  inert  bodies,  thereby  ren- 
dering them  temporarily  active  (induced 
activity).  The  atomic  transformations  cease 
only  when  the  electrons  constitute  stable 
atoms;  that  is,  a  non-radio-active  substance. 
For  example,  let  us  consider  uranium. 
Crookes  (79)  and  Becquerel  (80)  separated 
from  this  substance  an  active  and  a  non- 
active  part.  The  former,  which  Crookes  and 
Rutherford  call  uranium-X,  in  time  loses  its 
activity,  while  the  latter  slowly  acquires  it. 
This  inactive  uranium  little  by  little  becomes 


104  RADIO-ACTIVITY 

transformed  into  uranium-X,  and  the  trans- 
formation is  accompanied  by  radiation.  At 
the  same  time  the  uranium-X  becomes  trans- 
formed into  an  unknown  substance.  A  sim- 
ilar thing  happens  in  the  case  of  thorium, 
with  this  difference,  however,  that  in  addi- 
tion to  a  transformation  of  inactive  thorium 
into  thorium-X  accompanied  by  the  emission 
of  the  usual  rays,  there  is  the  production  of 
an  emanation  as  a  result  of  a  final  disintegra- 
tion of  the  atoms  which  constitute  the  tho- 
rium-X. The  emanation  itself  is  radio-active ; 
that  is,  able  to  transform  itself  into  other 
substances,  among  which  is  the  substance 
which  adheres  to  neighbouring  bodies  and 
causes  induced  radio-activity.  Also  in  the 
case  of  thorium  we  are  ignorant  of  what  is 
the  final  and  stable  atomic  condition. 

The  successive  transformations  of  the 
radium  atom  are  analogous  to  those  of  tho- 
rium, save  that  there  apparently  exists  no 
radium-X.  Thus  there  is  a  splitting  up  of 
the  radium  atoms  and  the  formation  of  an 
emanation,  or  perhaps  better  of  emanations, 
inasmuch  as  there  seem  to  exist  between  the 


RADIO-ACTIVITY  10$ 

radium  and  the  condensible  emanation  inter- 
mediate stages  of  transformation,- which  are 
the  transformations  taking  place  during  the 
emission  of  the  a,  /3,  and  y  rays.  The  ema- 
nation itself  is  radio-active,  and  in  under- 
going transformation  gives  rise  to  the 
substance  producing  induced  radio-activity; 
that  is,  to  a  substance  temporarily  radio- 
active and  thus  in  the  stage  of  further  trans- 
formation. Neither  in  the  case  of  radium 
nor  of  the  other  radio-active  bodies  are  we 
able  to  say  what  other  successive  aggrega- 
tions are  possible  before  a  stable  condition  is 
attained;  but  the  final  non-radio-active  sub- 
stance has  at  least  been  found  in  the  case  of 
radium.  It  has  in  fact  been  recognized  (81) 
that  in  time  the  emanation  of  radium  en- 
closed in  a  tube  becomes  modified ;  its  spec- 
trum slowly  changes  in  aspect  and  ends  by 
exhibiting  the  characteristic  lines  of  helium. 
This  gas,  recently  discovered  in  our  atmos- 
phere and  in  certain  minerals,  is  one  whose 
lines  are  found  in  the  solar  spectrum,  and 
hence  the  name  helium.  The  minerals  in 
which  it  is  found  are,  in  general,  those 


106  RADIO-ACTIVITY 

from  which  radium  is  obtained ;  a  fact  which 
is  naturally  explained  by  what  has  just  been 
mentioned.1 

A  very  recent  experiment  of  the  Curies 
and  Dewar  (82)  seems  to  establish  beyond 
all  doubt  the  transformation  of  the  radium 
emanation  into  helium.  About  four  deci- 
grams of  pure  and  dry  bromide  of  radium 
stood  for  three  months  in  a  small  flask  com- 
municating with  a  Geissler  tube.  This  was 
provided  with  two  electrodes  fused  through 
the  glass  to  allow  the  passage  of  an  electric 
discharge,  by  means  of  which  the  gas  con- 
tained in  the  tube  could  be  rendered  lumi- 
nous. At  first  a  good  vacuum  was  produced 
in  both  flask  and  tube,  and  it  was  noted 
that  the  radio-active  substance  continuously 
evolved  a  gas  which  amounted  to  a  cubic 
centimeter  per  month  (under  ordinary  press- 
ure). On  passing  a  discharge  through  the 

1  E.  Rutherford  stated,  in  a  paper  read  before  the  Interna- 
tional Congress  of  Arts  and  Sciences  in  St.  Louis  (September 
19-25,  1904),  that  the  evidence  at  present  indicates  that  the 
a-particle  is  helium.  See  the  important  work  by  Rutherford 
entitled  "Radio-activity,"  recently  published  in  the  Cam- 
bridge  Physical  Series.  -TRANSLATOR. 


RADIO-ACTIVITY  IO/ 

tube  and  on  examining  the  spectrum  of  the 
light  produced  in  this  manner,  the  hydrogen 
and  mercury  lines  were  seen,  the  latter  being 
due  to  the  fact  that  a  mercury  pump  was 
employed  in  obtaining  the  vacuum.  Thus 
far  no  trace  of  helium  had  appeared. 

The  same  bromide  of  radium  was  then 
placed  in  a  quartz  tube  and  heated  until  the 
salt  fused.  While  the  heating  lasted,  a  pump 
was  kept  working  in  order  to  draw  the  gas, 
which  was  being  given  off,  into  a  tube  cooled 
by  liquid  air. 

In  this  manner  about  2.6  cubic  centimeters 
of  gas  were  collected,  which  was  luminous 
on  account  of  the  emanation  it  contained. 
When  this  gas  was  brought  into  a  Geissler 
tube,  it  showed  only  the  spectrum  of  nitro- 
gen ;  this  was  still  the  case  after  an  attempt 
had  been  made  to  eliminate  the  nitrogen  by 
means  of  condensation  produced  by  cooling 
with  liquid  hydrogen.  At  this  point  the 
quartz  tube  containing  the  radium  salt  was 
hermetically  sealed  while  it  was  being  pumped 
out.  Twenty  days  later,  when  the  gas  con- 
tained in  the  tube  was  rendered  luminous  by 


108  RADIO-ACTIVITY 

means  of  a  discharge  between  external  elec- 
trodes of  tin-foil,  the  spectroscope  clearly 
showed  the  entire  spectrum  of  helium. 

After  this  it  seems  difficult  to  doubt  that 
helium  is  formed  as  a  result  of  the  radio- 
active transformations  of  radium.  Never- 
theless, many  other  proofs  of  this  character 
will  be  necessary  before  the  transformation 
of  the  chemical  atoms  may  be  considered  as 
a  demonstrated  fact ;  that  is  to  say,  before 
the  idea  of  the  absolute  invariability  of  the 
atoms,  which  with  time  has  become  so  deeply 
rooted  in  our  minds,  is  abandoned. 


CHAPTER   VI 

MASS,    VELOCITY,   AND    ELECTRIC    CHARGE    OF 
THE  IONS   AND  OF  THE  ELECTRONS 

THE  time  has  now  come  to  give  some 
idea  of  the  methods  by  which  it  has  been 
possible  to  measure  not  only  the  ratio  be- 
tween the  electric  charge  and  the  mass  of 
the  electrons  or  of  the  ions  and  the  separate 
values  of  these  two  quantities,  but  also  the 
velocity  with  which  they  move  under  various 
conditions.  Such  methods  for  the  most  part 
are  based  upon  the  effects  which  electric  and 
magnetic  fields,  either  acting  separately  or 
simultaneously,  produce  on  the  moving  elec- 
trified particles;  or  they  are  based  on  the 
heat  which  is  developed  by  the  particles 
themselves  when  they  strike  an  obstacle ;  or, 
finally,  on  the  property  which  the  particles 
possess  of  acting  as  nuclei  for  the  condensa- 
tion of  vapours.  Without  entering  into  the 
details  of  the  experiments  and  of  the  calcula- 

109 


1 10    MASS,   VELOCITY,   AND   ELECTRIC    CHARGE 

tions  which  relate  to  them,  let  us  examine 
these  different  phenomena. 

First,  let  us  consider  the  effect  which  the 
magnetic  field  produces  on  cathode  rays, 
which  move  so  as  to  render  the  wall  of  the 
discharge  tube  luminous  at  a  point  opposite 
the  cathode,  and  whose  direction  is  perpen- 
dicular to  that  of  the  field.  For  example, 
the  tube  in  which  the  cathode  rays  are  gen- 
erated may  be  placed  between  the  poles  of 
a  magnet.  The  electrons  in  motion,  which 
constitute  the  cathode  rays,  deviate  from 
their  rectilinear  path,  and  the  rays  them- 
selves assume  the  form  of  circular  arcs.  In 
fact,  the  magnetic  field  produces  an  electro- 
magnetic force  which  affects  the  moving 
electron.  This  force  is  in  the  direction  of 
that  which  the  field  would  produce  on  an 
electric  current  coinciding  with  the  trajec- 
tory, and  hence  it  is  perpendicular  both  to 
the  trajectory  and  to  the  magnetic  force ;  it 
has  no  effect  on  the  speed  of  the  electron, 
but  only  on  the  direction  of  its  motion,  which 
is  circular  and  uniform.  The  electromag- 
netic force  must  then  be  equal  and  opposite 


MASS,   VELOCITY,   AND   ELECTRIC   CHARGE     ill 

to  the  centrifugal  force  of  this  circular  mo- 
tion. Now  the  centrifugal  force  depends  in 
a  simple  and  known  manner  on  the  mass  and 
the  velocity  of  the  particle  and  on  the  radius 
of  its  trajectory ;  and,  on  the  other  hand,  the 
electromagnetic  force  is  proportional  to  the 
charge  of  the  electron  and  to  its  velocity, 
since  the  product  of  these  two  factors  repre- 
sents the  intensity  of  the  equivalent  electric 
current.  We  thus  arrive  at  a  simple  relation 
between  the  following  quantities:  first,  charge 
and  mass  of  the  electron,  or,  more  exactly,  the 
ratio  between  the  two ;  second,  the  velocity 
of  the  electron;  third,  the  intensity  of  the 
magnetic  field ;  fourth,  the  radius  of  the  cir- 
cular arc  which  is  described.1  The  last  two 
quantities  may  be  measured,  and  it  is  only 
necessary  to  know  one  of  the  others  in  order 
to  calculate  the  remaining  one. 

If  one  assumes,  as,  in  fact,  at  first  it  was 
assumed,  that  the  velocity  of  the  electron  in 

1  Let  e  represent  the  charge  of  each  particle,  m  its  mass, 
V  its  velocity,  H  the  intensity  of  the  magnetic  field,  p  the 
radius  of  the  trajectory.  The  relation  which  exists  between 

a 

these  quantities  is  V—  Hp — . 


112    MASS,   VELOCITY,   AND   ELECTRIC    CHARGE 

the  cathode  rays  is  of  the  order  of  the  mag- 
nitude of  the  molecular  velocities  in  gases, 
one  finds,  for  the  ratio  between  charge  and 
mass,  a  value  of  an  order  similar  to  that 
which  holds  for  ions  in  electrolysis.  Thus 
one  would  be  led  to  suppose  that  the  mass 
of  the  electron  is  of  the  order  of  magnitude 
of  that  of  the  material  atom.  Fortunately,  it 
was  recognized  very  soon  that  this  assump- 
tion was  incorrect,  and,  in  addition  to  the 
magnetic  deviation  of  the  cathode  rays, 
other  effects  were  sought  and  other  theo- 
retical considerations  were  undertaken  which 
might  lead  to  a  simultaneous  determination 
of  the  velocity  and  of  the  ratio  mentioned 
above.  For  example,  it  may  be  assumed 
that  the  velocity  of  the  electron  is  such  that 
its  kinetic  energy  must  equal  the  electric 
work  done  in  the  passage  of  the  charge  of 
the  electron  from  the  potential  of  the  cathode 
to  that  of  the  anode.  Such  a  method  as  this 
was  adopted  not  long  ago  by  Kaufmann  (83) 
and  Simon  (84). 

J.  J.  Thomson  (85)  determined  the  velocity 
of  the  electrons  by  measuring  the  negative 


MASS,   VELOCITY,  AND   ELECTRIC   CHARGE     113 

charge  which  they  gave  up  on  entering  a 
hollow  conductor  connected  with  an  elec- 
trometer, after  having  suffered  a  deviation 
in  a  magnetic  field,  and  by  measuring  at 
the  same  time,  by  means  of  a  thermo-elec- 
tric couple,  the  energy  transported  by  them. 
Thus  two  more  relations  may  be  added  to 
the  relation  which  exists  between  the  quanti- 
ties mentioned  above.1  One  of  these  states 
that  the  electric  charge  transported  to  the 
conductor  (this  is  found  by  direct  measure- 
ment) is  equal  to  the  number  of  electrons 
multiplied  by  the  constant  charge  of  each  of 
them  ;  the  other  states  that  the  energy  trans- 
ported and  transformed  into  heat  by  impact 
(and  measured  by  means  of  the  thermo-elec- 
tric couple)  is  equal  to  the  number  of  elec- 
trons multiplied  by  the  kinetic  energy  of 
each,  which  is  the  half-product  of  the  mass 
and  the  square  of  the  velocity. 

To  the  two  unknown  quantities  which  we 
had  at  first,  we  have  thus  added  a  new  one, 


tn 
1  The  two  relations  are  Q  =  We,         •  —  N=  W,  where  Q  is 


the  total  quantity  of  electricity  transported,  N"  the  number  of 
electrons,  and  IV  their  kinetic  energy. 


114    MASS,  VELOCITY,  AND  ELECTRIC   CHARGE 

which  is  the  number  of  electrons  set  in 
motion  during  the  experiment ;  but  now  we 
have  three  relations  instead  of  the  single 
one  we  had  at  first;  hence  by  eliminating 
the  number  of  electrons  it  becomes  possible 
to  calculate  both  the  ratio  between  the  charge 
and  mass  of  each  electron,  and  the  velocity 
as  well.1  The  first  results  of  any  importance 
were  obtained  by  this  method.  Values  of 
the  ratio  between  charge  and  mass  of  the 
electrons  were  obtained,  which  differed  but 
slightly  from  each  other  when  the  rarefied 
gas  in  which  the  cathode  rays  were  produced 
was  changed  (air,  hydrogen,  carbonic  acid 
gas).  In  every  case  the  value  found  clearly 
indicated  that,  if  every  electron  represents  an 
electric  charge  equal  to  that  of  an  electrolytic 
ion,  its  mass  must  be,  on  the  contrary,  very 
much  smaller  than  that  of  an  ion  of  hydro- 
gen. The  values  found  for  the  velocity  of 
the  electrons  showed,  that  it  is  very  much 

1  The  elimination  of  N  from  the  three  equations  leads  to 
the  two  following  :  — 

v_  ?.W  e  _    2  W 

QHp  m 

which  serve  to  calculate  V  and  — 

m 


MASS,   VELOCITY,  AND   ELECTRIC    CHARGE     11$ 

greater  than  the  molecular  velocities  of 
gases,  and  that,  in  fact,  it  was  in  this  in- 
stance about  one-tenth  of  the  velocity  of 
light. 

Another  method  for  the  determination  of 
the  velocity  of  the  electrons  of  the  cathode 
rays,  which  is  based  on  the  deviation  pro- 
duced by  an  electric  field,  is  also  due  to 
Thomson  (86). 


FIG.  14. 

The  cathode  rays  leave  the  cathode  C 
(Fig.  14)  and  are  reduced  to  a  narrow  bun- 
dle by  two  thick  metallic  diaphragms  A  and 
B,  communicating  with  the  earth  and  pro- 
vided with  narrow  horizontal  apertures,  and 
then  pass  between  two  metal  plates  D,  E. 
If  these  are  oppositely  charged,  the  rays 
should  deviate  from  their  rectilinear  path, 
since  the  negative  electrons,  which  consti- 


Il6    MASS,   VELOCITY,   AND   ELECTRIC    CHARGE 

tute  the  rays,  will  be  attracted  by  the  posi- 
tive plate  and  repelled  by  the  other.  Hertz 
had  been  unable  to  obtain  the  expected  devia- 
tion of  cathode  rays  in  an  electric  field,  and 
neither  did  Thomson  succeed  in  obtaining  it 
at  first,  because  the  conductivity  which  a 
rarefied  gas  acquires  when  cathode  rays  pass 
through  it  makes  it  impossible  to  maintain 
the  two  plates  at  a  sufficient  difference  of 
potential.  But  when  the  highest  attainable 
rarefaction  of  the  gas  is  reached,  it  is  possible 
to  obtain  the  effect  and  to  see  the  displace- 
ment of  the  luminous  spot  at  the  end  of  the 
tube,  due  to  the  phosphorescence  excited  by 
the  cathode  rays.  If,  for  example,  the  plate 
E  is  positive  and  D  negative,  the  luminous 
spot  descends,  which  shows  that  the  path  of 
the  electrons  has  been  curved  downward. 

In  fact,  the  electric  force,  which  may  be 
assumed  to  be  constant  if  the  two  plates  are 
sufficiently  large  and  near  together,  will  cause 
each  electron  to  describe  a  parabola,  and  thus 
the  cathode  rays  will  curve,  precisely  as  a  jet 
of  water  issuing  from  a  horizontal  tube  curves 
under  the  influence  of  gravity. 


MASS,   VELOCITY,  AND   ELECTRIC   CHARGE     117 

In  the  case  of  gravity  the  force  is  propor- 
tional to  the  mass  of  the  moving  body,  and 
the  acceleration  is  independent  of  the  mass. 
Here,  on  the  contrary,  the  force  acting  on  an 
electron,  and  hence  also  the  acceleration 
which  it  produces,  is  proportional  to  the  elec- 
tric charge;  and  since,  for  a  given  charge, 
and  hence  for  a  given  electric  force,  the 
acceleration  is  in  inverse  ratio  to  the  mass, 
it  may  be  said  that  the  acceleration  itself, 
on  which  the  form  of  the  parabola  depends 
in  a  known  manner,  is  proportional  to  the 
ratio  between  the  charge  and  the  mass  of 
the  electron.  Just  as  in  the  case  of  the  mag- 
netic deviation,  so,  also,  in  this  case  of  the 
electric  deviation  there  exists  a  relation 
which  contains  the  above  ratio  and  the  ini- 
tial velocity  of  the  electron.  Hence,  if  simul- 
taneously with  the  electric  field  a  magnetic 
field  is  produced,  the  lines  of  force  of  which 
are  perpendicular  both  to  the  lines  of  elec- 
tric force  and  the  direction  of  the  rays,  it 
will  be  possible  to  calculate  not  only  the 
ratio  between  the  charge  and  mass  of  each 
electron,  but  also  its  velocity.  Moreover,  it 


Il8    MASS,   VELOCITY,   AND   ELECTRIC    CHARGE 

is  possible  to  arrange  matters  in  such  a  way 
that  the  effects  due  to  the  two  fields  shall 
neutralize  each  other.  Then  the  two  quan- 
tities to  be  determined  may  be  deduced  from 
the  intensity  of  each  of  the  fields,  and  from 
the  deviation  produced  when  only  one  of 
them  acts.1  In  this  manner  Thomson  ob- 
tained a  value  for  the  velocity  about  one- 
tenth  that  of  light,  and  a  value  for  the  ratio 
between  the  charge  and  mass  of  an  electron 
in  agreement  with  that  found  by  the  other 
method. 

Similar  measurements  were  made  by  Mr. 
H.  A.  Wilson  (87),  who  used  a  series  of 
cathodes  made  of  different  metals  and  proved 
that  the  results  obtained  did  not  depend  on 
the  nature  of  the  cathode. 

Almost  at  the  same  time  Thomson's 
method  was  used  by  Lenard  (88),  who  ex- 

1  Denoting  by  0  the  deviation  produced  separately  either 
by  the  electric  or  by  the  magnetic  field,  by  /  the  length  of  the 
path  along  which  the  electron  is  exposed  to  the  deflecting 
forces,  by  F  the  intensity  of  the  electric  field,  and  by  H  the 
magnetic  field  strength,  we  have 

V=F--  L 

H '  m 


MASS,   VELOCITY,   AND   ELECTRIC    CHARGE     119 


perimented  with  the  rays  which  bear  his 
name;  that  is  to  say,  with  cathode  rays  which 
are  made  to  issue  through  thin  aluminium 
foil  from  the  tube  in  which  they  are  pro- 
duced. 

Lenard  also  (89)  applied  a  new  method,  in 
which  the  electrons  were  subjected  to  the 
action  of  an  electric  field  parallel  to  the 


FIG.  15. 


direction  of  their  motion.  For  this  purpose 
the  Lenard  rays  generated  by  the  cathode 
C  (Fig.  15)  issue  from  a  hole  A,  which  is 
closed  with  aluminium  foil,  and  thus  pene- 
trate into  the  apparatus  V,  containing  a 
highly  rarefied  gas.  In  this  apparatus  a  con- 
denser is  placed  consisting  of  two  parallel 
metallic  disks,  a,  b,  and  having  a  small  open- 
ing at  the  centre,  through  which  the  rays 


120    MASS,  VELOCITY,  AND   ELECTRIC   CHARGE 

coming  from  a  and  directed  toward  the 
phosphorescent  screen  S,  must  pass.  Two 
metallic  tubes,  m,  n,  protect  the  rays  from 
the  electric  action  of  the  disk  a  which  may 
be  insulated  and  charged,  while  b  is  kept 
always  in  communication  with  the  earth. 
Hence  it  is  only  in  the  path  between  a  and 
b  that  the  electrons  are  exposed  to  an  elec- 
tric force  which  increases  or  diminishes  their 
velocity  according  as  the  disk  a  is  positively 
or  negatively  charged.  These  same  rays 
are,  however,  subjected  farther  on  to  a  de- 
flecting force,  as  in  the  preceding  method, 
produced  by  a  magnetic  field  or  by  a  trans- 
verse electric  field  caused  by  the  parallel 
disks  d,  e,  and  the  deviation  thus  produced 
is  measured. 

As  one  might  expect,  for  a  given  electric 
field  between  d  and  e  the  deviation  depends 
upon  the  sign  of  the  charge  on  a,  since  the 
velocity  with  which  the  electrons  arrive  in 
the  field,  producing  the  deviation,  depends 
on  the  sign  of  this  charge.  By  measuring 
this  deviation  and  the  intensity  of  the  two 
fields  the  ratio  so  often  referred  to  may  be 


MASS,   VELOCITY,   AND   ELECTRIC   CHARGE     121 

calculated  ;  the  value  which  Lenard  obtained 
by  this  method  will  be  given  farther  on. 

Finally,  I  will  cite  a  most  ingenious, 
but  rather  complicated,  method  which  was 
adopted  by  Wiechert  (90)  in  making  a  di- 
rect and  accurate  determination  of  the  veloc- 
ity of  the  cathode  rays.  By  combining  his 
results  with  those  obtained  by  the  method 
of  magnetic  deviation,  he  was  able  to  calcu- 
late also  the  ratio  between  charge  and  mass 
of  the  electron. 

But  it  is  not  only  on  the  electrons  of  the 
cathode  rays  that  measurements  designed  to 
determine  their  characteristic  constants  have 
been  performed  ;  other  measurements  have 
been  made  both  on  the  negative  electrons 
emitted  by  metals  subjected  to  ultraviolet 
rays  and  on  those  emitted  by  incandescent 
or  by  radio-active  bodies.  Thus,  J.  J. 
Thomson  (91)  resorted  to  an  experimental 
method  in  which  a  magnetic  field  was 
caused  to  act  upon  the  electrons  emitted 
by  an  illuminated  metal.  By  means  of  an 
almost  identical  experimental  arrangement, 
the  writer  (92)  had  previously  determined 


122    MASS,  VELOCITY,   AND   ELECTRIC   CHARGE 


that  a  magnetic  field  reduces  the  transport 
of  negative  electricity  from  the  body  which 
receives  the  ultraviolet  rays  to  neighbour- 
ing bodies ;  Thomson 
interpreted  this  fact 
in  terms  of  the  new  the- 
ory, and  availed  himself 
of  it  in  the  following 
manner. 

A  small,  negatively 
charged  zinc  disk  AB 
(Fig.  1 6),  supported  by 
a  metallic  rod  Lt  is 
placed  in  a  vessel  con- 
taining highly  rarefied 
air  in  such  a  way  that 
it  may  be  brought  more 
or  less  close  to  a  metal 
grating  CZ?,  which  is 
parallel  with  the  disk 
and  connected  with  an 
electrometer.  The  vessel  is  closed  by  a 
quartz  plate  EF,  in  order  that  the  more 
refrangible  ultraviolet  radiations  caused  by  a 
shower  of  sparks  passing  between  zinc  wires 


FIG.  16. 


MASS,   VELOCITY,  AND   ELECTRIC   CHARGE     123 

may  not  be  absorbed  before  they  arrive  at 
the  charged  disk.  When  the  apparatus  has 
been  tested  to  make  sure  that  the  electrom- 
eter becomes  discharged  by  the  radiations, 
even  when  the  disk  AB  is  relatively  far  from 
the  grating,  a  magnetic  field  is  produced  the 
lines  of  force  of  which  are  parallel  to  the 
disk  and  to  the  grating.  It  is  then  found 
that  if  the  distance  between  the  disk  and  the 


FIG.  17. 

grating  is  increased  to  more  than  a  certain 
amount,  the  transport  of  the  negative  elec- 
tricity from  AB  to  CD  ceases  almost  entirely. 
The  mechanism  to  which  the  phenomenon 
is  due  is  the  following :  before  the  magnetic 
field  is  set  up  the  negative  electrons  which 
the  radiations  expel  from  the  zinc  disk  AB 
(Fig.  17)  pass  directly  from  AB  toward 
CD ;  but  after  a  magnetic  field  perpendicular 
to  the  plane  of  the  figure  is  produced,  each 
electron  describes  a  curve  which  may  be 
shown  to  be  a  cycloid,  and  as  a  result  the 


124    MASS,  VELOCITY,   AND   ELECTRIC  CHARGE 

electron  after  having  gone  a  certain  distance 
from  the  disk  returns  to  it  without  having 
been  able  to  reach  the  grating  if  the  latter 
is  too  far  distant.1  A  few  of  the  paths  de- 
scribed by  the  electrons  are  shown  as  broken 
lines  in  the  figure. 

The  maximum  distance  attained  by  an 
electron  depends  in  a  known  manner  upon 
the  intensity  of  the  electric  field  between 
the  disk  and  the  grating,  upon  the  intensity 
of  the  magnetic  field,  and  on  the  ratio  be- 

1  Choosing  the  J^-axis  in  the  direction  perpendicular  to 
the  disk  and  the  grating,  the  K-axis  perpendicular  to  the  X- 
axis  and  to  the  magnetic  field,  we  have  as  the  equations  of 
motion  :  — 

md^=Fe_HedjL 
dt*  dt" 


dP  dt 

Supposing  the  initial  values  of  x,  y,  —,  —  ,  to  be  zero,  the 
solution  of  these  equations  is  :  — 

x=  a  (i  —  cos  £/),  y  =  a  (bt  —  sin  bt),  in  which 


The  radius  of  the  circle  which  generates  the  cycloid  is  a,  and 
hence  the  maximum  distance  from  the  zinc  disk  which  an 
electron  may  attain  is  :  — 

2^L 
H** 


MASS,   VELOCITY,   AND  ELECTRIC   CHARGE     125 

tween  the  charge  and  mass  of  the  electron. 
Hence  it  is  possible  to  determine  this  ratio 
by  measuring  the  first  two  quantities  and  the 
maximum  distance  between  the  disk  and  the 
grating  beyond  which  the  latter  no  longer 
receives  a  sensible  charge.  The  accuracy  of 
the  determination  is  limited  by  the  fact  that 
the  electrons  do  not  all  leave  the  disk  with 
equal  velocities,  and  hence  the  maximum 
distance  from  it  which  they  attain  is  not  the 
same  for  all. 

Thomson  (93)  applied  a  similar  method 
to  the  electrons  emitted  by  an  incandescent 
metal.  Lenard  (94)  measured  the  ratio  by 
resorting  to  the  action  of  ultraviolet  rays  on 
metals  in  a  vacuum;  and  finally  Becquerel 
(95)  and  others  determined  the  constants  for 
electrons  emitted  by  radio-active  bodies. 
Without  at  present  entering  into  the  details 
of  these  last  determinations  and  of  others 
which  have  given  analogous  results,  it  will 
be  well  to  tabulate  the  principal  values  of  the 
ratio  between  the  charge  and  mass  of  an  elec- 
tron in  order  that  these  values  may  be  com- 
pared with  each  other. 


126    MASS,   VELOCITY,  AND   ELECTRIC    CHARGE 


SOURCE  OF 
THE  ELEC- 
TRONS 

EXPERIMENTER 

DATE 

METHOD  USED 

RATIO 

BETWEEN 

CHARGE  AND 
MASS 

Cathode 

J.J.Thomson 

1897 

Electric  Deviation  and 

231  •  io15* 

Rays 

Magnetic  Deviation. 

Cathode 

J.J.Thomson 

1897 

Magnetic  Deviation, 

351  •  io« 

Rays 

charge  transported, 

and  heat  evolved,  j 

Cathode 

Kaufmann 

1897- 

Magnetic  Deviation, 

558-101* 

Rays 

1898 

and 

Potential  Difference. 

Lenard 

Lenard 

1898 

Electric  Deviation  and 

191.7.  lo16 

Rays 

Magnetic  Deviation. 

Lenard 

Lenard 

1898 

Deviation  and  Electric 

204  •  io18 

Rays 

Field. 

Cathode 

Simon 

1899 

Magnetic  Deviation 

559-5  •  Io15 

Rays 

and 

Potential  Difference. 

Cathode 

Weichert 

1899 

Magnetic  Deviation 

(303  -  io« 

Rays 

and  Velocity. 

\46s  •  io« 

Ultra- 

J.J.Thomson 

1899 

Diminution  of  the 

228  -  io16 

violet 

discharge  by  the  action 

Rays 

of  a  magnetic  field. 

Metal 

J.J.Thomson 

1899 

Diminution  of  the 

261  •  io* 

heated  to 

discharge  by  the  action 

Redness 

of  a  magnetic  field. 

Ultra- 

Lenard 

1900 

Magnetic  Deviation 

345.10^ 

violet 

and 

Rays 

Electric  Field. 

/3-Rays 

Becquerel 

1900 

Electric  Deviation 

about 

from 

and 

300-  io16 

Radium 

Magnetic  Deviation. 

*  231- io18  signifies  the  number  obtained  by  writing  13  zeros  to  the  right  of  the 
231;  that  is  to  say,  the  number  231,000,000,000,000,000.  The  unit  of  charge 
adopted  here  is  the  electrostatic,  which  is  defined  as  the  quantity  of  electricity 
which  repels  a  like  charge  placed  at  a  distance  of  one  centimeter  with  unit  force; 
that  is,  with  the  force  of  one  dyne. 


MASS,  VELOCITY,  AND  ELECTRIC  CHARGE    127 

If  account  be  taken  of  the  great  variety 
of  phenomena  in  which  the  negative  elec- 
trons make  themselves  manifest,  and  of  the 
diversity  of  the  methods  adopted  in  the 
measurement  of  the  ratio  between  the  charge 
and  the  mass  of  each  electron,  the  accord 
between  the  results  is  most  remarkable. 
There  can  then  remain  no  doubt  as  to  the 
order  of  magnitude  of  this  ratio  since  it  turns 
out  to  be  between  663  and  1937  times  greater 
than  the  analogous  ratio  for  the  hydrogen  ion 
in  electrolysis,  which  is  equal  to  0.289- io15, 
and  still  greater  than  that  which  relates 
to  the  ions  of  other  bodies.  Therefore,  the 
particles  constituting  the  cathode  rays  and 
the  /8-rays  from  radio-active  bodies  cannot 
be  atoms,  but  must  be  particles  of  very  much 
smaller  mass.  Thus  it  is  that  the  existence 
of  masses  which  are  much  smaller  than  that 
of  the  smallest  of  the  atoms  of  known  sub- 
stances, has  been  demonstrated  in  the  surest 
possible  manner  and  by  purely  physical 
methods. 

The  difference  between  the  values  found 
by  various  investigators  for  the  ratio  between 


128    MASS,   VELOCITY,   AND   ELECTRIC  CHARGE 

the  charge  and  mass  of  the  electrons  is  not 
solely  due  to  errors  in  measurement,  since 
the  very  accurate  experiments  made  by  Kauf- 
mann  (96)  prove  that  this  ratio  varies  with 
the  velocity  of  the  electrons,  becoming  rapidly 
smaller  as  the  velocity  approaches  that  of 
light.  Kaufmann  caused  a  magnetic  and  an 
electric  field  to  act  on  the  /3-rays,  emitted  by 
one  of  the  salts  of  radium,  in  such  a  way 
that  the  fields  had  a  common  direction  per- 
pendicular to  that  of  the  rays.  The  action 
of  the  magnetic  force  is  to  cause  the  rays  to 
deviate  in  a  certain  direction,  while  that  of 
the  electric  force  is  to  cause  the  rays  to  devi- 
ate in  a  direction  at  right  angles  to  this. 
Thus  the  experimental  arrangement  resem- 
bles the  well-known  one  of  the  crossed 
prisms,  in  which  two  prisms  cause  light  to 
undergo  two  successive  deviations  at  right 
angles  to  each  other.  Just  as  in  this  latter 
experiment  each  coloured  ray,  separated  from 
white  light  by  the  action  of  the  first  prism, 
suffers  a  new  deviation  due  to  the  second 
prism,  and  these  deviations  may  be  sepa- 
rately measured,  so  also  in  Kaufmann's  ex- 


MASS,   VELOCITY,   AND   ELECTRIC   CHARGE     129 

periment  the  deviation  of  each  of  the  /3-rays, 
which  differ  as  regards  the  velocity  of  the 
electrons  which  constitute  them,  may  be 
measured. 

By  such  a  method  as  this  Kaufmann  found 
for  the  ratio  between  the  charge  and  mass  of 
the  electrons  values  approximately  equal  to 
those  obtained  by  other  experimenters  so  long 
as  he  was  dealing  with  electrons  of  relatively 
low  velocity,  or  electrons  constituting  feebly 
penetrating  /2-rays,  but  he  found  much 
smaller  values  for  the  more  penetrating  rays ; 
in  fact,  the  ratio  fell  off  to  about  one-half  of 
the  usual  value  for  electrons  whose  velocity 
was  about  nine-tenths  that  of  light. 

Inasmuch  as  everything  seems  to  point  to 
the  fact  that  the  electric  charge  is  always  the 
same  for  all  electrons,  it  must  be  held  that 
their  mass  is  not  constant,  but  increases 
rapidly  as  their  velocity  approaches  that  of 
light.  This  result  is  of  great  importance, 
since  it  is  in  conformity  with  the  suppo- 
sition that  the  electrons  do  not  possess  a 
material  mass  in  the  ordinary  sense  of  the 
word,  but  only  an  apparent  mass  due  to  the 


130    MASS,   VELOCITY,   AND   ELECTRIC    CHARGE 

fact  that  they  are  electric  charges  in  motion. 
But  we  shall  return  to  this  subject  in  the  last 
chapter. 

Wien  (97)  and  Thomson  (98)  carried  out 
measurements  on  the  positive  ions  which 
constitute  the  canal  rays,  similar  to  those  on 
the  negative  electrons.  The  first  of  these 
physicists  obtained  as  a  result  a  velocity  of 
3600  kilometers  per  second,  and  the  value 
0.009. i ot5  for  the  ratio;  Thomson  found  the 
ratio  to  be  o.oi2-io15.  Thus  it  is  certain  that 
the  positive  particles  in  the  canal  rays  are 
not  electrons,  but  are,  instead,  atoms  or  per- 
haps heavy  electrified  atomic  groups. 

The  preceding  conclusion  relative  to  the 
small  mass  of  the  electrons  is  based  on  the 
hypothesis  that  the  electric  charge  of  the  ions 
in  gases  is  equal  to  that  which  is  associated 
with  every  ion  in  electrolysis  (or  better,  with 
every  valency  of  the  ion).  From  now  on 
this  may  be  considered  an  established  fact 
rather  than  a  hypothesis.  In  fact,  the  study 
of  the  diffusion  of  ions  in  gases  (99)  has  led 
to  the  important  conclusion  that  the  electric 
charge  of  each  ion  is  sensibly  equal  to  that 


MASS,  VELOCITY,   AND   ELECTRIC    CHARGE     131 

which  the  ions  have  in  electrolysis.  But 
even  before  this  result  had  been  obtained 
Thomson  (100)  measured  directly  the  charge 
of  each  ion  in  a  gas,  employing  an  extremely 
clever  method  of  which  we  will  now  attempt 
to  give  an  idea. 

When  air  saturated  with  water  vapour  sud- 
denly expands,  a  part  of  the  vapour  condenses 
in  a  cloud  as  a  result  of  the  cooling  which 
accompanies  the  expansion,  and  each  little 
globule  generally  has  as  a  nucleus  a  particle 
of  the  microscopic  dust  which  is  ordinarily 
contained  in  the  atmosphere.  It  seems,  in 
fact,  as  though  the  presence  of  very  minute 
bodies  was  required  for  the  condensation  of 
vapour,  and  that  the  condensation  begins  on 
the  surface  of  these  bodies  whose  radius  of 
curvature  must  certainly  be  very  small.  At 
any  rate,  the  presence  of  these  corpuscles  is 
favourable  to  the  liquefaction  of  the  vapour ; 
for  if  the  dust  is  carefully  removed,  it  is 
necessary  to  expand  the  moist  air  considera- 
bly more  in  order  that  the  cloud  may  form. 

Mr.  C.  T.  R.  Wilson  (101)  showed  that  a 
gas  containing  ions  behaves  like  air  charged 


132    MASS,   VELOCITY,   AND   ELECTRIC    CHARGE 

with  dust,  since  the  ions  themselves  act  as 
nuclei  or  centres  of  condensation  for  the 
vapour.  And,  in  fact,  an  expansion  of  the 
air  which  may  be  too  small  to  cause  a  cloud 
to  form,  may,  on  the  contrary,  be  sufficient 
to  precipitate  the  vapour  when  the  air  itself 
is  ionized ;  for  example,  by  passing  Rontgen 
rays  through  it.  That  the  change  in  be- 
haviour of  the  gas  is  really  due  to  the  pres- 
ence of  ions  was  confirmed  by  the  fact  that 
the  formation  of  the  cloud  is  again  retarded, 
or  what  amounts  to  the  same  thing,  a  greater 
expansion  is  necessary,  when  the  ions  are 
removed  by  the  passage  of  an  electric  cur- 
rent through  the  gas. 

The  apparatus  used  by  Thomson  is  some- 
what complicated,  but  the  essential  part  is  a 
vessel  which  contains  the  moist  air,  and  in 
which  two  horizontal  conductors  are  placed 
one  above  the  other.  The  upper,  which  may 
serve  to  close  the  vessel,  generally  consists 
of  a  disk  of  thin  aluminium  which  is  kept 
in  electrical  connection  with  the  earth ;  the 
lower,  which  may  consist  of  the  mass  of  water 
which  keeps  the  air  above  it  saturated,  is  con- 


MASS,  VELOCITY,  AND  ELECTRIC  CHARGE    133 

nected  to  an  electrometer.  At  the  proper 
moment  the  air  is  ionized  by  allowing  X-rays 
or  the  rays  emitted  by  a  radio-active  body  to 
enter  the  vessel  by  passing  through  the  alu- 
minium plate.  Ions  may  also  be  introduced 
into  the  vessel  by  charging  the  lower  con- 
ductor negatively  and  by  letting  ultraviolet 
rays  fall  on  its  surface.  In  this  case  the  alu- 
minium disk  is  replaced  by  a  metallic  grat- 
ing, the  vessel  is  closed  by  a  quartz  plate, 
and  a  zinc  or  other  metallic  disk  is  chosen 
as  the  lower  conductor. 

In  the  first  part  of  the  experiment  a  meas- 
urement is  made  of  the  quantity  of  electricity 
which  passes  from  one  conductor  to  the  other 
in  unit  time ;  that  is  to  say,  the  intensity  of 
the  current  which  traverses  the  ionized  air  is 
measured.  This  is  found  by  multiplying  the 
diminution  of  potential  in  unit  time,  which 
is  calculated  from  the  decrease  in  the  devia- 
tion of  the  electrometer,  by  the  capacity  of 
the  system  formed  by  the  lower  conductor 
and  the  electrometer.  But,  on  the  other 
hand,  this  current  is  due  to  the  motion  of 
the  ions  under  the  action  of  the  electric  field 


134    MASS,   VELOCITY,  AND   ELECTRIC   CHARGE 

which  exists  between  the  two  disks ;  and  the 
intensity  of  the  current  may  be  calculated 
in  terms  of  the  following  quantities :  the 
strength  of  the  electric  field,  the  number  of 
ions  which  exist  in  unit  volume,  the  charge 
of  each  ion,  and  the  velocity  with  which  they 
move  from  one  disk  to  the  other.  By  equat- 
ing the  two  values  for  the  current  strength  a 
relation  is  obtained  which  contains  the  num- 
ber of  ions,  their  charge  and  their  velocities, 
in  addition  to  the  numerical  data  resulting 
directly  from  the  experiment.1  But  the  ve- 
locity of  the  ions  in  an  electric  field  of  known 

1  Calling  the  intensity  of  the  field  between  the  two  disks 
E,  the  number  of  ions  per  cubic  centimeter  JV,  the  velocity 
of  the  ions  in  a  field  of  unit  intensity  F,  the  charge  of  each 
e,  the  area  of  the  disks  A,  the  intensity  of  the  current  from 
one  disk  to  the  other  is  obviously 

NVEeA; 

and  if  C  is  the  capacity  of  the  insulated  disk,  P  the  diminu- 
tion of  its  potential  in  unit  time,  the  equation  which  is  men- 
tioned in  the  text  is 

NVEeA  =  CP. 

By  measuring  E,  A,  C,  F,  and  by  assuming  for  V  the  value 
given  by  special  experiments  which  Rutherford  undertook  to 
determine  this  velocity  (480  centimeters  per  second  in  the 
case  of  ions  produced  in  air  by  X-rays),  only  A7" and  e  remain 
unknown.  N  was  measured  in  the  manner  described  farther 
on  in  the  text. 


MASS,  VELOCITY,   AND   ELECTRIC    CHARGE     135 

intensity  has  been  measured  and  is  known ; 
hence,  in  order  to  arrive  at  a  determination 
of  the  charge  of  the  ions,  only  the  number 
which  exist  in  unit  volume  remains  to  be 
measured. 

The  second  part  of  the  experiment  serves 
to  accomplish  this.  If  a  cloud  is  produced 
by  a  suitable  expansion  of  the  air,  it  is  suffi- 
cient to  count  in  some  way  the  globules  of 
water  of  which  it  is  formed  in  order  that  the 
object  of  the  experiment  may  be  attained. 
In  fact,  assuming  that  each  of  the  ions  is  a 
nucleus  of  one  of  the  drops,  the  number  of 
the  former  would  equal  the  number  of  the 
latter.  Now,  to  estimate  the  number  of 
drops,  it  is  sufficient  to  divide  the  total  mass 
by  the  mass  of  each  drop.  The  calculation  of 
the  total  mass  is  based  on  the  measurement  of 
the  lowest  temperature  which  the  air  reaches 
in  the  act  of  expansion  and  on  the  measure- 
ment of  the  temperature  of  the  air  itself  after 
the  formation  of  the  cloud ; 1  the  mass  of 
each  drop  is  inferred  from  its  diameter,  and 

1  Calling  the  two  temperatures  /,  /',  the  specific  heat  of  the 
air  at  constant  volume  C,  the  latent  heat  of  vaporization  of 


136    MASS,   VELOCITY,   AND   ELECTRIC  CHARGE 

this  in  turn  from  the  velocity  with  which  it 
falls,  or  from  the  rate  of  descent  of  the  upper 
surface  of  the  cloud,  which  is  always  suffi- 
ciently sharp  in  outline.  Naturally  a  rela- 
tion exists  between  the  dimensions  of  a 
sphere  and  the  velocity  with  which  it  falls. 
While  it  is  true  that  all  bodies  fall  with  equal 
velocity  in  a  vacuum,  this  is  not  the  case  in 
air,  the  resistance  of  which  more  or  less  slows 
down  their  motion.  In  the  case  of  a  sphere 
this  slowing  down  is  the  greater,  the  smaller 
the  sphere,  because,  while  the  weight  is  pro- 
portional to  the  volume  and  hence  to  the 
cube  of  the  diameter  of  the  sphere,  the  re- 
sistance offered  by  the  air  is  proportional  to 
the  surface,  and  hence  to  the  square  of  the 
diameter.  Therefore,  as  this  is  diminished, 
the  weight  falls  off  more  rapidly  than  the 
resistance  of  the  air.  This  is  the  reason  why 
bodies  reduced  to  fragments,  such  as  powders 
and  small  drops,  fall  so  slowly  that  they  often 
seem  to  stand  still. 

the  water  Z,  the  mass  of  the  air  in  unit  volume  M,  the  mass 
of  vapour  condensed  in  a  cubic  centimeter  of  air  g,  we  have 

Lq=CM(t'  -/). 


MASS,  VELOCITY,   AND    ELECTRIC   CHARGE     137 

To  determine  the  diameter  of  the  drops  of 
water,  a  formula1  is  employed  which  contains 
this  diameter  in  terms  of  the  velocity  of 
descent  and  the  viscosity  of  the  medium  in 
which  the  particles  move.  The  number  of 
ions  and  therefore  the  charge  of  each  may 
be  calculated  in  the  manner  described  above 
when  the  diameter  is  known. 

The  final  result  which  Thomson  obtained 
from  a  series  of  observations  made  on  air 
ionized  by  X-rays  was  that  the  charge  of 
each  ion  is  (in  electrostatic  units)  6.5  .  io~10, 
or  o.ooo  ooo  ooo  65.  He  deduced  from  his 
other  experiments  on  the  action  of  ultra- 
violet rays  on  zinc  the  value  of  6.8  •  io~10, 
which  differs  only  very  slightly  from  the 
above  result. 

However,  Thomson  (102)  himself  later 
found  that  a  correction  should  have  been 
applied  to  these  values.  It  has  been  proved 
that  the  condensation  of  vapour  on  the  nega- 

1  If  V  is  the  velocity  with  which  a  drop  falls,  r  its  radius, 
ft.  the  coefficient  of  viscosity  of  the  air,  and  g  the  acceleration 
of  gravity,  this  formula  is  :  — 

9  n  y=  2  grz. 
For  air  ju,  =  0.00018. 


138     MASS,   VELOCITY,   AND   ELECTRIC   CHARGE 

tive  ions  takes  place  with  an  expansion  less 
than  the  minimum  necessary  in  the  case  of 
condensation  on  positive  ions,  and  that  prob- 
ably, in  the  experiment  described,  a  few  of  the 
positive  ions  acted  as  nuclei.  In  fact,  it  is 
now  known  that,  when  the  volume  of  the 
moist  air  is  suddenly  increased  in  the  ratio 
of  i  to  1.25,  the  negative  and  not  the  positive 
ions  act  as  nuclei  in  the  formation  of  the 
drops;  it  is  only  when  the  expansion  is  in 
the  ratio  of  i  to  1.31  that  the  positive  ions 
begin  to  take  part  in  the  phenomenon. 

A  series  of  new  experiments,  in  which 
account  was  taken  of  this,  led  to  the  more 
accurate  result  of  3.4  •  io~10  electrostatic  units 
as  the  charge  of  each  single  ion. 

Shortly  after  this,  new  research  on  the 
same  subject  was  undertaken  by  Mr.  H.  A. 
Wilson  (103),  who  adopted  a  slightly  different 
method,  which  has  the  advantage  of  not  re- 
quiring the  evaluation  of  the  number  of  ions 
existing  in  the  air  subjected  to  the  Rbntgen 
rays. 

In  this  case  also,  by  a  sudden  expansion  of 
the  air,  the  water  vapour  is  condensed  into 


MASS,   VELOCITY,   AND   ELECTRIC   CHARGE     139 

drops  having  an  ion  as  a  nucleus,  but  the 
cloud  is  made  to  form  between  two  parallel 
metallic  disks.  When  these  are  oppositely 
electrified,  the  velocity  with  which  the  drops 
fall  is  different  from  that  which  they  pos- 
sessed before  the  electric  field  was  set  up 
between  the  disks.  Whether  the  change  is 
an  increase  or  decrease  depends  on  the  direc- 
tion of  the  electric  force.  By  measuring  this 
velocity,  sufficient  data  are  available  for  the 
calculation  of  the  charge  of  each  drop.1 

Without  entering  into  further  details  rela- 
tive to  these  very  ingenious  experiments,  I 
will  point  out  that  they  confirm  the  fact, 

1  Calling  the  velocity  with  which  the  drops  fall  when  there 
is  no  electric  field  z/,  that  when  the  field  accelerates  the  fall 
v",  the  intensity  of  the  field  X,  the  charge  of  the  ion  e,  the 
mass  of  a  drop  m,  the  acceleration  of  gravity  g,  it  is  evident 

that  we  have 

mgv"  =  (mg  +  Xe)  T/  ; 

but  from  the  equation 


of  the  preceding  footnote  we  may  deduce 
m  =  3.14  •  lo~9  'is'*. 
Hence,  on  eliminating  m,  we  have 


which  permits  a  calculation  of  e  based  on  the  measurement  of 
X,  v',  and  v". 


previously  noted  by  Thomson,  that  when  the 
intensity  of  the  X-rays  employed  to  ionize  the 
air  is  very  great,  a  number  of  ions  may  unite 
and  form  the  nucleus  of  a  single  drop.  In 
this  case  it  follows  that  the  charge  of  the 
drop  is  either  twice,  or  three  times,  etc.,  that 
of  a  drop  which  has  but  a  single  ion  as  a 
nucleus.  Wilson's  experiments  show  that 
this  really  may  happen,  since  the  cloud  sepa- 
rates into  several  horizontal  layers  which  fall 
with  different  velocities.  Evidently  one  of 
the  layers  is  formed  by  drops  having  but  one 
ion  as  a  nucleus,  another  layer  by  those  hav- 
ing two  ions  as  a  nucleus,  etc.  It  is  clear 
that,  by  taking  this  into  account,  a  measure- 
ment of  the  velocity  with  which  any  one  of 
the  various  layers  falls  may  be  used  in  cal- 
culating the  quantity  sought.  The  result 
which  Wilson  obtained  (3.1  •  io~10)  differs  but 
slightly  from  Thomson's  result.  This  value 
is  almost  identical  with  that  found  by  calcu- 
lating the  charge  of  the  hydrogen  ion  in  elec- 
trolysis, assuming  the  mass  of  the  ion  to  have 
the  value  calculated  from  the  kinetic  theory 
of  gases. 


MASS,   VELOCITY,   AND   ELECTRIC    CHARGE     141 

Thus  we  see  that  the  numerical  agreement 
which  the  theory  would  lead  one  to  expect 
is  substantiated  within  the  limits  of  precision, 
which  in  this  sort  of  research  may  at  present 
be  reasonably  demanded. 


CHAPTER   VII 

THE   ELECTRONS   AND    THE   CONSTITUTION   OF 
MATTER 

ON  account  of  the  facility  with  which  the 
electron  theory  lends  itself  to  the  formation 
of  a  model  of  the  mechanism  of  physical 
phenomena,  it  possesses  a  decided  utility, 
even  in  the  minds  of  those  who  see  in  it 
only  an  instrument  for  research.  In  reality, 
the  theory  itself  is  hardly  in  its  initial  stages, 
and  it  would  be  premature  to  consider  it 
just  yet  as  a  solid  basis  for  a  new  system 
of  natural  philosophy.  Nevertheless,  since 
even  from  this  point  of  view  it  is  constantly 
acquiring  importance,  it  will  be  useful  to 
dedicate  this  last  chapter  to  a  concise  expo- 
sition of  the  hypothesis,  according  to  which 
matter  is  now  coming  to  be  considered  as 
built  up  of  electrons. 

A  role  of  fundamental  importance  is 
assigned  to  the  electrons  in  this  new  mode 

142 


THE  CONSTITUTION   OF  MATTER         143 

of  conceiving  the  constitution  of  bodies ;  but 
in  order  that  it  may  be  possible  to  explain 
the  known  phenomena  in  terms  of  the  elec- 
trons, it  is  necessary  to  suppose  them  to  be 
endowed  with  certain  essential  properties. 
Thus,  for  example,  it  is  held  that  there  exist 
electrons  of  two  sorts,  which  are  in  a  certain 
sense  mutually  antagonistic,  namely,  negative 
electrons  and  positive  electrons ;  it  is  held 
that  the  first  and  not  the  second  may  exist 
in  the  free  state;  it  is  admitted  that  the 
separation  of  a  negative  electron  takes  place 
more  easily,  or  with  less  expenditure  of 
energy,  from  certain  atoms,  like  those  of 
the  metals,  than  from  others.  However, 
the  fundamental  property  attributed  to  the 
electrons  is  that,  in  their  essence,  they  con- 
sist of  electric  charges  acting  on  each  other 
in  the  manner  expressed  by  the  formulae  of 
Hertz  or  Maxwell.  It  follows  from  this  that 
the  new  theory  does  not  pretend  to  give  a 
reason  for  the  cause  of  electric  phenomena. 
This  still  remains  a  mystery.  While  for- 
merly, starting  with  the  existence  of  cosmic 
ether  and  that  of  ponderable  matter,  char- 


144         THE   CONSTITUTION   OF   MATTER 

acterized  by  its  principal  attribute,  inertia, 
the  attempt  was  made  to  give  a  mechanical 
explanation  for  all  phenomena ;  now,  on  the 
contrary,  starting  with  the  ether  and  the 
electrons,  the  attempt  is  made  to  construct, 
so  to  speak,  ponderable  matter  out  of  these 
and  to  take  account  of  the  phenomena 
which  it  presents.  Hence  it  may  be  said 
that  the  electron  theory  is  much  more  a 
theory  of  matter  than  a  theory  of  electricity ; 
or,  rather,  in  the  new  system  electricity  is 
set  up  in  the  place  of  matter,  the  existence 
of  which  was,  on  the  whole,  not  much  better 
understood  than  is  the  essence  of  the  elec- 
trons at  the  present  time. 

In  order  to  better  understand  the  impor- 
tance of  the  hypothesis  and  the  fundamental 
attributes  of  the  electrons,  it  will  be  well  for 
us  to  consider  synthetically  the  phenomena 
due  to  electrified  bodies,  either  at  rest  or  in 
motion.  Following  a  line  of  reasoning  due 
to  Lodge  (104),  let  us  suppose  that  we  bring 
into  contact  two  bodies  of  different  material 
and  then  separate  them.  They  soon  present 
that  group  of  properties  which  constitute  the 


THE   CONSTITUTION   OF  MATTER          145 

two  opposite  electric  states  and,  in  particular, 
they  attract  each  other  and  create  the  electric 
field  which  surrounds  them.  If  one  of  the 
two  bodies,  say  that  which  has  the  positive 
electric  properties,  is  removed  to  an  infinite 
distance,  only  the  negative  body  need  be 
taken  into  consideration.  If  we  also  suppose 
it  to  be  very  small,  the  electric  field  will  be 
represented  by  rectilinear  lines  of  force  arriv- 
ing at  the  body  from  all  directions.  The 
surrounding  ether  is  now  deformed,  giving 
to  this  word  the  very  broadest  meaning ;  that 
is  to  say,  it  exists  in  a  strained  condition, 
which  is  shown  not  only  by  the  tensions 
along  the  lines  of  force,  which  cause  the 
apparent  forces  at  a  distance,  but  also  by 
the  transverse  pressures.  What  the  cause  of 
this  special  state  of  the  ether  is,  how  it  can  be 
susceptible  of  a  dual  aspect,  that  is,  how  it 
can  correspond  to  a  positive  and  to  a  negative 
charge,  are  questions  which  we  are  absolutely 
unable  to  answer,  just  as  we  are  unable  to 
answer  the  question,  what  is  the  real  struc- 
ture and  real  nature  of  the  entity  which  exists 
everywhere  and  which  is  called  the  ether. 


146         THE   CONSTITUTION   OF  MATTER 

Let  us  now  suppose  that  the  small  nega- 
tively electrified  body  moves  with  uniform 
speed;  that  is,  we  will  suppose  the  special 
state  of  deformation  just  defined  changes  its 
position  in  the  ether.  It  may  be  deduced 
from  Maxwell's  theory  and  from  direct  ex- 
periment as  well,  that  this  propagation  of 
the  ethereal  strain  from  place  to  place  pro- 
duces the  magnetic  field.  This  may  then  be 
considered  to  be  due  to  a  deformation  differ- 
ent in  nature  from  the  electric  deformation 
but  analogous  to  it,  since  tensions  along  the 
lines  of  force  and  pressures  in  the  transverse 
direction  also  exist  in  the  magnetic  field. 
And  if  the  motion  is  rectilinear,  the  lines  of 
force  are  circles  with  their  centres  on  the 
trajectory  and  lying  in  the  planes  perpen- 
dicular to  this.  A  series  of  electrified  bodies 
following  each  other  with  uniform  motion  has 
the  properties  of  an  electric  current.  Thus  a 
constant  current  may  be  considered  as  a  flow 
of  equidistant  electrons  in  uniform  motion, 
and  a  variable  current  as  a  flow  of  electrons 
in  variable  motion,  or  of  electrons  which  do 
not  follow  one  another  at  equal  intervals. 


THE   CONSTITUTION   OF  MATTER          147 

If  the  small  electrified  body  moves  with 
non-uniform  motion,  the  magnetic  field  which 
it  creates  is  variable,  and  the  phenomenon 
of  induction  takes  place.  If  the  motion  is 
periodic,  the  phenomena  of  light  occur. 
Every  variation  in  the  velocity  of  electrified 
bodies  causes  a  variation  in  the  magnetic 
field,  this  produces  a  variation  in  the  electric 
field,  and  these  two  variations  are  propagated 
together  with  the  velocity  of  light. 

Let  us  now  suppose  that  at  a  given  mo- 
ment we  wish  to  increase  the  velocity  of  the 
electrified  body,  which  we  will  suppose  to 
move  up  to  this  moment  with  uniform  speed. 
On  account  of  the  relations  existing  between 
the  electric  and  magnetic  force  in  an  electro- 
magnetic field,  it  is  not  possible  to  accelerate 
the  motion  of  the  electrified  body  without 
the  expenditure  of  energy.  In  fact,  an  in- 
crease in  the  velocity  results  in  a  variation 
of  the  magnetic  field,  which  in  turn  produces 
an  electric  force  tending  to  oppose  the  accel- 
eration of  the  motion.  In  the  same  way  a 
decrease  in  the  velocity  is  opposed  by  the 
generation  of  an  electric  force  which  tends 


148         THE   CONSTITUTION   OF  MATTER 

to  conserve  the  velocity  of  the  electrified 
particle.  In  both  cases  the  electromagnetic 
phenomenon  is  such  as  to  simulate  inertia, 
and  the  body  by  the  mere  fact  of  its  being 
electrified  behaves  as  though  its  mass  were 
larger  than  it  really  is. 

What  has  been  said  of  the  small  electrified 
body  holds  for  an  electron,  and  its  mass, 
which  we  have  stated  to  be  less  than  one- 
thousandth  that  of  the  hydrogen  atom,  is  at 
least  in  part  not  real,  but  apparent. 

This  species  of  apparent  inertia,  which  an 
electrified  body  or  an  electron  presents,  is  a 
manifestation  of  the  phenomenon  called  self- 
induction  in  the  case  of  electric  currents.  In 
fact,  if  there  are,  instead  of  a  single  moving 
electron,  a  great  number  of  such  electrons 
following  one  another  at  small  and  equal 
intervals  along  the  same  path,  these  repre- 
sent an  electric  current.  An  increase  or 
diminution  in  the  velocity  of  the  electrons 
gives  rise  to  an  increase  or  diminution  of  the 
number  which  pass  a  given  point  on  the  path 
in  unit  time,  and  hence  correspond  to  an 
increase  or  diminution  in  the  intensity  of  the 


THE  CONSTITUTION  OF  MATTER         149 

current.  Now  what  has  been  said  about  the 
effect  due  to  the  variation  in  the  velocity  of 
a  single  moving  electrified  body  or  of  a 
single  electron,  is  substantially  true  of  any 
number  of  electrons,  and  thus  each  variation 
in  their  velocity  generates  a  force  which  tends 
to  hinder  the  variation  itself.  Each  change 
in  the  intensity  of  the  current  generates  an 
electromotive  force,  which  tends  to  oppose 
the  change  or  to  produce  a  new  current  in 
such  a  direction  as  to  diminish  the  change. 
As  is  seen,  this  current  is  the  extra  current, 
and  the  electromotive  force  is  the  electromo- 
tive force  of  self-induction. 

In  summing  up,  it  may  be  said  that  the 
electrons  determine  the  production  of  the 
so-called  electrostatic  phenomena,  when  they 
are  stationary;  of  the  phenomena  of  magneto- 
statics  and  of  constant  currents  when  they 
are  in  uniform  motion ;  of  electromagnetic 
phenomena  when  they  move  non-uniformly, 
and  of  optical  phenomena  when  they  move 
with  periodic  motion.  A  sudden  variation 
in  the  velocity  of  an  electron  which  may  be 
due,  for  example,  to  a  collision,  generates  an 


150         THE   CONSTITUTION   OF   MATTER 

electromagnetic  wave  in  the  ether  analogous 
to  the  waves  caused  by  an  explosion  in  air. 
The  X-rays  are  the  manifestation  of  these 
waves. 

We  are  now  in  a  position  to  comprehend 
in  what  consists  the  modern  hypothesis, 
according  to  which  matter  is  built  up  of  elec- 
trons. First  of  all,  we  may  admit  that  the 
electrons  are  not  matter,  in  the  ordinary  sense 
of  the  word ;  that  is,  they  do  not  possess  mass 
other  than  that  which  they  seem  to  have  by 
reason  of  their  motion  and  electric  charge. 
Kaufmann's  experiment,  referred  to  in  the 
preceding  chapter,  renders  this  hypothesis 
very  probable.  He  found,  in  fact,  that  the 
ratio  between  the  charge  and  the  mass  of  the 
moving  electrons  increases  rapidly  as  the  ve- 
locity approaches  that  of  light.  And  since 
the  hypothesis  of  a  varying  charge  would 
be  too  improbable,  it  only  remains  to  be  sup- 
posed that  the  mass  rapidly  increases.  Now 
such  a  result  as  this  is  in  accord  with  the 
hypothesis  according  to  which  the  mass  of 
the  electrons  is  entirely  electromagnetic  in 
its  origin. 


THE   CONSTITUTION   OF   MATTER          151 

Nothing  prevents  us  from  supposing  that 
matter,  and  hence  all  known  bodies,  consist 
of  aggregations  or  systems  of  electrons  since 
the  electrons,  which  may  be  considered  as 
simple  electric  charges  devoid  of  matter  or 
as  consisting  in  a  modification  of  the  ether 
symmetrically  distributed  about  a  point,  per- 
fectly simulate  inertia  by  reason  of  the  laws 
of  the  electromagnetic  field  and  thus  show 
the  fundamental  property  of  matter. 

Therefore  it  may  be  admitted  that  a  mate- 
rial atom  is  nothing  but  a  system  consisting 
of  a  certain  number  of  positive  and  an  equal 
number  of  negative  electrons,  and  that  the 
latter,  or  at  least  some  of  them,  move  about 
the  remaining  portion  like  satellites.  Mo- 
lecular and  atomic  forces  would  then  be 
nothing  but  the  manifestations  of  the  elec- 
tromagnetic forces  of  the  electrons,  and  grav- 
itation itself  might  be  explained  with  these 
concepts  as  a  basis.  In  fact,  this  has  already 
been  attempted. 

If  we  suppose  one  or  more  negative  elec- 
trons to  be  taken  away  from  an  atom,  it  be- 
comes a  positive  ion,  while  the  addition  of 


152         THE   CONSTITUTION   OF   MATTER 

one  or  more  negative  electrons  to  a  neutral 
atom  produces  a  negative  ion.  The  manner 
in  which  various  bodies  behave  when  sub- 
jected to  free  electrons  in  motion,  as  in  the 
case  of  the  cathode  rays,  is  such  as  to  indorse 
the  hypothesis.  It  has  indeed  been  found 
that  a  body  prevents  the  passage  of  the  elec- 
trons, or  absorbs  the  cathode  rays,  in  propor- 
tion to  its  density;  that  is,  in  proportion  to 
the  total  number  of  electrons,  which  consti- 
tute it,  independently  of  the  manner  in  which 
they  are  grouped  to  form  chemical  atoms  of 
various  kinds. 

The  electrons  would  seem  to  be,  therefore, 
the  elements  of  construction  in  the  architec- 
ture of  the  atoms.  When  such  a  hypothesis 
as  this  is  once  adopted,  the  dogma  of  the 
invariability  of  the  chemical  atom  or  of  the 
impossibility  of  transmutation  of  chemical 
substances  is  forever  banished  from  science, 
since  according  to  this  hypothesis  everything 
is  built  up  of  electrons.  We  have  already 
seen  how  the  phenomena  of  radio-activity 
seem  to  show  transformations  of  this  kind. 

If,  in  addition,  it  is  held  that  all  bodies  are 


THE   CONSTITUTION   OF  MATTER        153 

at  least  slightly  radio-active  and  hence  emit 
ions  and  electrons,  these  new  views  relative 
to  the  structure  of  matter  become  quite  simi- 
lar to  those  which  were  advanced  as  the  basis 
of  a  general  explanation  of  physical  phenom- 
ena more  than  a  half-century  ago  by  a  clever 
and  original  Italian  physicist,  Ambrogio  Fu- 
sinieri  (105).  In  spite  of  the  fact  that  the 
concepts  of  this  physicist  have  hitherto  been 
open  to  objections  and  now  have  lost  a  part 
of  their  value  in  the  light  of  subsequently 
discovered  facts,  one  thinks  at  once  of  what 
he  called  attenuated  matter  emitted  by  all 
bodies,  when  one  speaks  of  emanations  sent 
out  by  radio-active  bodies,  or  of  electrons 
which,  like  a  species  of  slow  and  invisible 
evaporation,  are  probably  given  off  in  a  con- 
tinuous manner  by  every  material  substance, 
and  diffuse  into  surrounding  space. 


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(39)  H.   BECQUEREL.      Comp.   Rend.,   Vol.    CXXIX, 

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(45)  H.  BECQUEREL.     Comp.  Rend.,  October  27,  1903, 

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(47)  H.  BECQUEREL.     Comp.  Rend.,  January  25,  1904. 

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(50)  R.   J.    STRUCT.      Phil.    Mag.,   November,    1903, 

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(51)  P.  CURIE.     L'Electricien,  January  23,  1904. 

(52)  A.  HEYDWEILLER.     Phys.  Zeitschr.,  Vol.  IV,  p.  81 

(1902). 

(53)  E.  DORN.     Phys.  Zeitschr.,  Vol.  IV,  p.  530  (1902). 

(54)  P.  CURIE  ET  A.  LABORDE.     Comp.  Rend.,  Vol. 

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(55)  N.   GEORGIEWSKI,  Jour,  de  la  Soc.  Phys.  Chim. 

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(56)  A.    RIGHI.       Mem.   della    R.    Ace.   di   Bologna, 

Series  5,  Vol.  X,  p.  595  (1903). 

(57)  H.  BECQUEREL  ET  P.  CURIE.     Comp.  Rend.,  Vol. 

CXXXII,  p.  1289  (1901). 

(58)  J.  DANYSZ.     Comp.  Rend.,  Vol.  CXXXVI,  p.  461 


(59)  ASCHKINASS  UND   CASPARi.     Arch,  ftir  die  Ges. 

Physiol.,  Vol.  86  (1901). 

(60)  DANLOS.     Soc.  de  derm.,  November  7,  1901. 
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1902. 


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1904,  p.  130. 

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(66)  P.   ET   S.   CURIE.      Comp.  Rend.,  Vol.  CXXIX 

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(67)  ELSTER  UND  GEITEL.     Phys.  Zeitschr.,  Vol.  Ill, 

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(68)  A.    SELLA.     II  Nuovo  Cimento,  Vol.  3,  p.  138 ; 

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(69)  E.  RUTHERFORD  AND  H.  T.  BARNES.     Phil.  Mag., 

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(71)  J.  J.  THOMSON.     Nature,  February  26,  1903. 
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(73)  J-  J-  THOMSON.     Nature,  1903,  p.  90. 

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(75)  H.  S.  ALLEN.     Nature,  August  13,  1903. 

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1847. 


INDEX 


[THE  NUMBERS  REFER  TO  THE  PAGES.] 


o-rays,  73. 

Absorption  of,  74. 

Magnetic  deviation  of,  73. 

Nature  of,  73. 

Velocity  of,  74,  97. 
Actinium,  63. 
Anode  rays,  52. 
Aschkinass  and  Caspari : 

Effect  of  radium  on  bacteria,  90. 

/J-rays,  75. 

Absorption  of,  76. 

Magnetic  deviation  of,  75. 

Negative  charge  transported  by, 

76. 
Baskerville : 

Carolinium,  65. 
Becquerel  : 

Magnetic  deviation  of  a-rays,  73. 

Magnetic  deviation  of  (3-rays,  75. 

Rays,  58,  61. 

Theory  of  spinthariscope,  79. 
Bemont : 

Preparation  of  radium,  62. 

Canal  rays,  52. 
Carolinium,  65. 
Cathode : 

Dark  space,  31. 
Cathode  rays,  32. 

lonization  by,  48. 

Transport  of  charge  by,  35. 

Velocity  of,  35. 
Charge  of  electron  : 

Ratio  to  mass,  118,  121,  125. 

Variation  of  ratio,  128. 


Charge  of  ion,  131  etseq. 
Curie,  M. : 

Discharging   action  of  radium 

rays,  85. 
Curie,  Mme.  : 

Preparation  of  radium,  66. 

Radio-activity  of  thorium,  6x. 
Curie,  M.  and  Mme. : 

Charge  transported  by  |3-rays,  76. 

Preparation  of  polonium,  62. 
Curie,  M.  and  Mme.  and  Laborde : 

Evolution  of  heat  by  radium,  87. 
Crookes,  Sir  W. : 

Radiant  matter,  34. 

Spinthariscope,  79. 

Danysz : 

Physiological  effects  of  radium, 

90. 
Debierne : 

Preparation  of  actinium,  63. 
Dewar  and  Curie : 

Production  of  helium,  106  et  seq. 
Dissociation : 

Electric,  9. 

Electrolytic,  10. 
Dorn: 

Loss  of  weight  of  radium,  86. 

Magnetic  deviation  of  0-rays,  75. 

— ,  118,  121, 125,  130. 

M 

Variation  of,  128, 130. 
Electric  field  : 

Deviation  of  rays  by,  115  et  seq, 
Electric  shadow,  49. 


163 


1 64 


INDEX 


Electrolysis,  3. 
Electron : 

Charge  of,  26,  129. 

Mass  of,  37,  129,  150. 
Elster  and  Geitel : 

Excited  activity  in  atmosphere,95- 

Faraday : 

Dark  space,  30. 

Fourth  state  of  matter,  33. 
Fluorescence,  32. 

y-rays,  77. 
Georgiewski : 

Thermal  effect,  88. 
Giesel : 

Magnetic  deviation  of  0-rays,  75. 

Radio-lead,  63. 

Helium : 

Produced  from  radium  emana- 
tion, 106  et  seq. 
Heydweiller : 

Loss  of  weight  of  radium,  86. 
Hofmann  and  Strauss : 

Radio-lead,  63. 
Hofmann  and  Wolfl : 

Radio-lead,  64. 
Huggins,  Sir  W.  and  Lady : 

Spectrum  of  radium,  80. 

Ions: 

As  nuclei  of  condensation,  132. 
Charge,  mass,  and  velocity  of, 

13°,  137  ft  *eq- 

lonization,  40. 
By  impact,  45,  50. 
Causes  of,  45. 
Due  to  Lenard  rays,  47. 

Kaufmann : 

Variation  of  —   128,  150. 
m 

Lenard : 
— ,  118  et  seq. 


Transport  of  charge  by  cathode 

rays,  35. 
Lenard  rays  : 

lonization  by,  47. 
Lorenz  : 

Electromagnetic  theory  of  light, 


Magnetic  field  : 

Effect  on  a-rays,  73. 

Effect  on  P-rays,  75. 

Effect  on  cathode  rays,  no. 

Effect  on  circular  vibration,  20 

et  seq. 
Maiorana  : 

Velocity  of  cathode  rays,  35. 
Marckwald  : 

Radio-tellurium,  64. 
Meyer  and  Von  Schweidler  : 

Magnetic    deviation    of    P-rays, 
75- 

Perrin: 

Transport  of  charge  by  cathode 

rays,  35. 
Pitchblende,  62. 
Polonium,  62. 

Rays  emitted  by,  72. 

Radiant  matter,  34. 
Radiation  : 

From  radio-active  bodies,  69. 
Radio-active  bodies  : 

Chemical  actions  due  to,  82. 

Effects  produced  by,  78. 

Evolution  of  heat  by,  96. 

Loss  of  weight  by,  86. 
Radio-active  emanations,  91. 

Gaseous  nature  of,  92. 

Presence  in  the  atmosphere,  95. 
Radio-activity  : 

Induced,  65,  94. 

Of  snow,  100. 

Of  spring  water,  98. 
Radio-lead,  63. 
Radio-tellurium,  64. 


INDEX 


I65 


Radium,  63. 

Evolution  of  heat  by,  87. 

Physiological  effect  of  rays  from, 
90. 

Preparation  of,  65  et  seq. 

Spectrum  of,  68,  80. 
Rutherford : 

Induced  radio-activity,  94. 

Radio-active  emanations,  91  et 
seg. 

Thorium-X,  96. 

Transport  of  charge  by  a-rays,  73. 
Rutherford  and  Soddy : 

Theory  of  atomic  disintegration, 
102  et  seg. 

Saturation  current,  43. 
Schmidt : 

Radio-activity  of  thorium,  61. 
Secondary  rays,  85. 
Selenium : 

Effect  of  rays  from  radium  on,  91. 
Sella : 

Excited  activity  in  the    atmos- 
phere, 95. 
Spinthariscope,  79. 
Strutt: 

Nature  of  «-rays,  73. 

Production     of     electricity     by 
radium,  84. 


Theory  of  atomic  disintegration, 

102  et  seg. 
Thomson,  J.  J. 
Charge  of  the  ion,  131. 
Velocity  of  cathode  rays,  35. 
Velocity  of  the  electrons,  112  ei 
seq. 

Velocity  and  —  of  the  positive 

• 

ions,  130. 
Thorium,  61. 
Thorium-X,  96, 104. 

Uranium : 
Radio-active  compounds  of,  57, 

61. 
Uranium-X,  103. 

Weber : 

Atomic  theory  of  electricity,  5. 
Wien: 

Velocity  and  —  of  positive  ions, 
m 

130-    . 

Wilson,  C.  T.  R. 
Ions  as  nuclei  of  condensation, 

132. 
Wilson,  H.  A. 

Charge  of  the  ion,  138. 

Zeeman  effect,  14  et  seq. 


Principles  of  Inorganic  Chemistry 

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